Science

9.4 Increasing and Reducing In some situations, however, friction Friction is undesirable and we would want to minimise

9. 4 Increasing and Reducing In some situations, however, friction Friction is undesirable and we would want to minimise it. As you have seen in the previous section, Why do you sprinkle fine powder on friction is desirable in some situations. the carrom board (Fig. 9. 12)? You might Have you ever thought why the sole have noticed that when a few drops of of your shoe is grooved? It oil are poured on the hinges of a door, is done to provide the shoes better grip the door moves smoothly. A bicycle and on the floor, so that you can move safely. a motor mechanic uses grease between Similarly, the treaded tyres of cars, trucks the moving parts of these machines. In and bulldozers provide better grip with all the above cases, we want to reduce the ground. Powder is sprinkled on the carrom board to reduce friction friction in order to increase efficiency. When oil, grease or graphite is applied between the moving part of a machine, a thin layer is formed there and moving (a) (b) surfaces do not directly rub against each (a) Soles of shoes and (b) tyres are other (Fig. 9. 13). Interlocking of treaded to increase friction irregularities is avoided to a great extent. We deliberately increase friction by Movement becomes smooth. The using brake pads in the brake system substances which reduce friction are of bicycles and automobiles. When you called lubricants. In some machines, it are riding a bicycle, the brake pads do may not be advisable to use oil as not touch the wheels. But when you lubricant. An air cushion between the press the brake lever, these pads arrest moving parts is used to reduce friction. the motion of the rim due to friction. Surface 2 The wheel stops moving. You might have seen that kabaddi players rub their Lubricant hands with soil for a better grip of their opponents. Gymnasts apply some coarse Surface 1 substance on their hands to increase friction for better grip. Action of lubricant 2024-25


Science

[Activity] ## Activity 9. 4 Take a few pencils which are Can we reduce friction to cylindrical in shape. Place them zero by polishing surfaces parallel to each other on a table. or using large amount of Place a thick book over it lubricants? (Fig. 9. 15). Now push the book. You observe the pencils rolling as the book moves. Do you feel it easier to move the book in this way than to slide it? Do you think that Friction can never be entirely eleminated. No resistance to the motion of the book surface is perfectly smooth. has been reduced? Have you seen Some irregularities are heavy machinary being moved by always there. placing logs under it? 9. 5 Wheels Reduce Friction You must have seen attaches and other pieces of luggage fitted with rollers. Even a child can pull such pieces of luggage (Fig. 9. 14). Why is it so? Let us find out. Motion of the book on rollers When one body rolls over the surface of another body, the resistance to its motion is called rolling friction. Rolling reduces friction. It is always easier to roll than to slide a body over another. That is the reason it is convenient to pull luggage fitted with rollers. Can you now understand why the wheel is said to be one of the greatest inventions of mankind? Since the rolling friction is smaller than the sliding friction, sliding is replaced in most machines by rolling by the use of ball bearings. Common examples are the use of ball bearings between hubs and the axles of ceiling Rolling reduces friction fans and bicycles (Fig. 9. 16). 2024-25 is fluids. So we can say that fluids exert force of friction on objects in motion through them. The frictional force exerted by fluids is also called drag. The frictional force on an object in a fluid depends on its speed with respect to the fluid. The frictional force also depends on the shape of the object and the nature of the fluid. It is obvious that when objects move through fluids, they have to overcome friction acting on them. In this process they lose energy. Efforts are, therefore, made to minimise friction. So, objects are given special shapes. Where do you think scientists get hints for these special shapes? From nature, of course. Birds and fishes have to move about in fluids Ball bearings reduce friction all the time. Their bodies must have evolved to shapes which would make 9. 6 Fluid Friction them lose less energy in overcoming You know that air is very light and thin. friction. Look carefully at the shape Yet it exerts frictional force on objects of an aeroplane (Fig. 9. 17). Do you moving through it. Similarly, water and find any similarity in its shape and other liquids exert force of friction when that of a bird? In fact, all vehicles are objects move through them. In science, designed to have shapes which reduce the common name of gases and liquids fluid friction. Similarity in shapes of an aeroplane and a bird 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT BALL BEARING (cid: 220) Friction opposes the relative motion between two surfaces in contact. It acts on both the DRAG surfaces. (cid: 220) Friction depends on the nature of surfaces in FLUID FRICTION contact. FRICTION (cid: 220) For a given pair of surfaces friction depends upon the state of smoothness of those INTERLOCKING surfaces. (cid: 220) Friction depends on how hard the two surfaces LUBRICANTS press together. ROLLING FRICTION (cid: 220) Static friction comes into play when we try to move an object at rest. SLIDING FRICTION (cid: 220) Sliding friction comes into play when an object is sliding over another. STATIC FRICTION (cid: 220) Sliding friction is smaller than static friction. (cid: 220) Friction is important for many of our activities. (cid: 220) Friction can be increased by making a surface rough. (cid: 220) The sole of the shoes and the tyres of the vehicle are treaded to increase friction. (cid: 220) Friction is sometimes undesirable. (cid: 220) Friction can be reduced by using lubricants. (cid: 220) When one body rolls over another body, rolling A riddle for you friction comes into play. Rolling friction is smaller than sliding friction. In some situations, I oppose the motion in other situations, I facilitate the motion (cid: 220) In many machines, friction is reduced by but, I always oppose using ball bearings. the relative motion (cid: 220) Fluid friction can be minimised by giving between two moving surfaces. suitable shapes to bodies moving in fluids. Put some lubricant, and I become small there. Make the moving surfaces rough I make the movement tough. I may be- static, sliding or rolling but whenever two surfaces are in motion I am always there, Tell me who I am! 2024-25


Science

[Exercise] ## Exercises 1. Fill in the blanks. (a) Friction opposes the __ between the surfaces in contact with each other. (b) Friction depends on the __ of surfaces. - Friction produces __. - Sprinkling of powder on the carrom board __ friction. (e) Sliding friction is __ than the static friction. 2. Four children were asked to arrange forces due to rolling, static and sliding frictions in a decreasing order. Their arrangements are given below. Choose the correct arrangement. (a) rolling, static, sliding (b) rolling, sliding, static - static, sliding, rolling - sliding, static, rolling 3. Alida runs her toy car on dry marble floor, wet marble floor, newspaper and towel spread on the floor. The force of friction acting on the car on different surfaces in increasing order will be (a) wet marble floor, dry marble floor, newspaper and towel. (b) newspaper, towel, dry marble floor, wet marble floor. - towel, newspaper, dry marble floor, wet marble floor. - wet marble floor, dry marble floor, towel, newspaper 4. Suppose your writing desk is tilted a little. A book kept on it starts sliding down. Show the direction of frictional force acting on it. 5. You spill a bucket of soapy water on a marble floor accidently. Would it make it easier or more difficult for you to walk on the floor? Why? 6. Explain why sportsmen use shoes with spikes. 7. Iqbal has to push a lighter box and Seema has to push a similar heavier box on the same floor. Who will have to apply a larger force and why? 8. Explain why sliding friction is less than static friction. 9. Give examples to show that friction is both a friend and a foe. 10. Explain why objects moving in fluids must have special shapes. 2024-25 Extended Learning - ## Activities and Projects 1. What role does friction play in the sport of your choice? Collect some pictures of that sport in action where friction is either supporting it or opposing it. Display these pictures with proper captions on the bulletin board of your classroom. 2. Imagine that friction suddenly vanishes. How would life be affected. List ten such situations. 3. Visit a shop which sells sports shoes. Observe the soles of shoes meant for various sports. Describe your observations. 4. A toy to play with: Take an empty match box. Take out its tray. Cut a used refill of a ball pen of the same width as the tray as shown in the figure below. Fix the refill with two pins on the top of the tray as shown in Fig. 9. 18. Make two holes on the opposite sides of the tray. Make sure that the holes are large enough to allow a thread to pass through them easily. Take a thread about a metre long and pass it through the holes as shown. Fix beads at the two ends of the thread so that it does not come out. Insert the tray in the outer cover of the matchbox. Suspend the match box by the thread. Leave the thread loose. The match box will start falling down due to gravity. Tighten the thread now and observe what happens. Explain your observation. Can you relate it to friction? Fig. 9. 18 You can read more on the related topic on the following websites: - http: //ww. school-for-champions. com/science/friction. htm - http//hyperphysics. phy-astr. gsu. edu/hbase/firct2. html 2024-25


Science

SS OOUUNNDD H ow do you come to know that a Sound plays an important role in our 'period' is over in your school? lives. It helps us to communicate with You come to know easily that one another. We hear a variety of sounds someone is at your door when he knocks in our surroundings. Make a list of sounds you hear in or you hear the sound of the doorbell. your surroundings. Most of the time you can make out that In the music room of your school you someone is approaching you by just hear the sounds produced by musical hearing the foot steps. instruments like flute, tabla, You might have played a game called harmonium, etc. (Fig. 10. 1). hide and seek. In this game a person is How is sound produced? How does it blind-folded and has to catch the travel from one place to another? How remaining players. How is the blind- do we hear sound? Why are some sounds folded person able to guess which player louder than others? We shall discuss is closest to her? such questions in this chapter. Tabla Sitar Flute Harmonium Some musical instruments 2024-25


Science

10.1 Sound is Produced by a Activity 10.2 Vibrating Body Take a rubber band. Put it around Touch the school bell when

[Activity] 10. 1 Sound is Produced by a ## Activity 10. 2 Vibrating Body Take a rubber band. Put it around Touch the school bell when not in use. the longer side of a pencil box What do you feel? Again touch it when (Fig. 10. 3). Insert two pencils producing sound. Can you feel it between the box and the stretched vibrating? rubber. Now, pluck the rubber band somewhere in the middle. Do you


Science

Activity 10.1 hear any sound? Does the band

[Activity] ## Activity 10. 1 hear any sound? Does the band vibrate? Take a metal plate (or a pan). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick (Fig. 10. 2). Do you hear a sound? Touch the plate or pan gently with your finger. Do you feel the vibrations? Plucking the rubber band As you learnt in Class VII the to and fro or back and forth motion of an object is called vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.


Science

[Activity] ## Activity 10. 3 Take a metal dish. Pour water in it. Strike it at its edge with a spoon (Fig. 13. 4). Do you hear a sound? Again strike the dish and then touch it. Can you feel the dish vibrating? Strike the dish again. Striking a pan Look at the surface of water. Do you Again strike the plate with the stick see any waves there? Now hold the and hold it tightly with your hands dish. What change do you observe immediately after striking. Do you on the surface of water? Can you still hear the sound? Touch the explain the change? Is there a hint plate after it stops producing sound. to connect sound with the Can you feel the vibrations now? vibrations of a body? 2024-25 Musical Instruments and their Vibrating Parts S. No. Musical Vibrating Part Instrument Producing Sound 1. Veena Stretched string 2. Tabla Stretched membrane 3. Vibrating dish produces waves 4. in water 5. We see that a vibrating object 6. produces sound. In some cases, the 7. vibrations are easily visible to us. But in most cases, their amplitude is so small Many of you might have seen the that we cannot see them. However, we manjira (cymbals), the ghatam, and the can feel them. noot (mudpots) and the kartal. These


Science

Activity 10.4 musical instruments are commonly

[Activity] ## Activity 10. 4 musical instruments are commonly used in many parts of our country. These Take a hollow coconut shell and instruments are simply beaten or struck make a musical instrument ektara. (Fig. 10. 6). Can you name a few other You can also make it with the help musical instruments of this type? of an earthen pot (Fig. 10. 5). Play You too can make a musical this instrument and identify its instrument. vibrating part. Ghatam Ektara Make a list of familiar musical instruments and identify their vibrating Manjira parts. A few examples are given in Table 10. 1. Complete rest of the Table. A few more musical instruments 2024-25


Science

[Activity] ## Activity 10. 5 Take 6-8 bowls or tumblers. Fill them with water up to different When we speak, does levels, increasing gradually from any part of our body one end to the other. Now take vibrate? a pencil and strike the bowls gently. Strike all of them in succession. You 10. 2 Sound Produced by will hear pleasant sounds. This is Humans your jaltrang (Fig. 10. 7). Speak loudly for a while or sing a song, or buzz like a bee. Put your hand on your throat as shown in Fig. 10. 8. Do you feel any vibrations? In humans, the sound is produced by the voice box or the larynx. Put your fingers on the throat and find a hard bump that seems to move when you swallow. This part of the body is known as the voice box. It is at the upper end of the windpipe. Two vocal cords, are stretched across the voice box or larynx in such a way that it leaves a narrow slit between them for the passage of air (Fig. 10. 8). Jaltrang When we pluck the string of an instrument, like the sitar, the sound that we hear is not only that of the string. The whole instrument is forced Voice box in humans to vibrate, and it is the sound of the vibration of the instrument that we When the lungs force air through the hear. Similarly, when we strike the slit, the vocal cords vibrate, producing membrane of a mridangam, the sound sound. Muscles attached to the vocal that we hear is not only that of the cords can make the cords tight or loose. membrane but of the whole body of When the vocal cords are tight and thin, the instrument. the type or quality of voice is different 2024-25 from that when they are loose and The vocal cords in men are about thick. Let us see how the vocal cords 20 mm long. In women these are about function. 15mm long. Children have very short vocal cords. This is the reason why


Science

Activity 10.6 the voices of men, women and

[Activity] ## Activity 10. 6 the voices of men, women and children are different. Take two rubber strips of the same size. Place these two pieces one 10. 3 Sound Needs a Medium above the other and stretch them for Propagation tight. Now blow air through the gap between them. As the When you call up your friend who is air blows through the stretched standing at a distance, your friend is rubber strips, a sound is produced. able to hear your voice. How does the You can also take a piece of paper sound propagate or travel to her? with a narrow slit and hold it between your fingers as shown in ## Activity 10. 7 Fig. 10. 9 (b). Now blow through the Take a metal or glass tumbler. Make slit and listen to the sound. Our sure that it is dry. Place a cell phone vocal cords produce sound in a in it. (## Remember that the cell phone similar manner. must not be kept in water. ) Ask your friend to give a ring on this cell phone from another cell phone. Listen to the ring carefully. Now, surround the rim of the tumbler with your hands (Fig. 10. 10). Put your mouth on the (a) (b) Sound needs a medium to travel Fig. 10. 9 (a), (b): Working of vocal cords 2024-25 opening between your hands. Place your ear gently on the water Indicate to your friend to give a ring surface (Fig. 10. 11). (Be careful: the again. Listen to the ring while water should not enter in your ear. ) sucking air from the tumbler. Can you hear the sound of the bell? Does the sound become fainter Does it indicate that sound can as you suck air? travel through liquids? Remove the tumbler from your mouth. Does the sound become loud again? Oh! That is how whales Can you think of an explanation? Is and dolphins might be it possible that the decreasing amount communicating under water. of air in the tumbler had something to do with decreasing loudness of the ring? Let us find out if sound can travel Indeed, if you had been able to suck through solids also. all the air in the tumbler, you will not listen any sound. Actually, sound needs


Science

[Activity] ## Activity 10. 9 a medium to travel. When air has been removed completely from a vessel, it is said Take a metre scale or a long metal that there is a vacuum in the vessel. The rod and hold its one end to your sound cannot travel through a vacuum. ear. Ask your friend to gently Does sound travel in liquids? Let us scratch or tap at the other end of find out. the scale (Fig. 10. 12).


Science

[Activity] ## Activity 10. 8 Take a bucket or a bathtub. Fill it with clean water. Take a small bell in one hand. Shake this bell inside the water to produce sound. Make sure that the bell does not touch the body of the bucket or the tub. Sound travelling through a metre scale Can you hear the sound of the scratching? Ask your friends around you if they were able to hear the same sound? Sound travelling through water 2024-25 You can also perform the above 10. 4 We Hear Sound through activity by placing your ear at one end Our Ears of a long wooden or metallic table and The shape of the outer part of the ear is asking your friend to gently scratch the like a funnel. When sound enters it, it other end of the table (Fig. 10. 13). travels down a canal at the end of which there is a thin stretched membrane. It is called the eardrum. It performs an important function. To understand what the eardrum does, let us build a tin-can model of the eardrum.


Science

[Activity] ## Activity 10. 10 Take a plastic or tin-can. Cut its ends. Stretch a piece of rubber Sound can travel through solids balloon across one end of the can We find that sound can travel and fasten it with a rubber band. through wood or metal. In fact, sound Put four or five grains of dry cereal can travel through any solid. You can on the stretched rubber. Now ask perform interesting activities to show your friend to speak "Hurrey, that sound can also travel through Hurrey" from the open end strings. Have you ever made a toy (Fig. 10. 15). Observe what happens telephone (Fig. 10. 14). Can you say that to the grain. Why do the grains sound can travel through strings? jump up and down? A toy telephone We have learnt so far that vibrating objects produce sound and it is carried in all directions in a medium. The medium could be a gas, a liquid or a Understanding action of an eardrum solid. How do we hear it? 2024-25 The eardrum is like a stretched You have already learnt in earlier classes rubber sheet. Sound vibrations make about the oscillatory motion and its the eardrum vibrate (Fig. 10. 16). The time period. eardrum sends vibrations to the inner The number of oscillations per ear. From there, the signal goes to the second is called the frequency of brain. That is how we hear. oscillation. Frequency is expressed in hertz. Its symbol is Hz. A frequency of 1 Hz is one oscillation per second. If an object oscillates 20 times in one second, what would be its frequency? You can recognise many familiar sounds without seeing the objects producing them. How is it possible? These sounds must be different to enable you to recognise them. Have you Inner ear ever thought what factors make them different? Amplitude and frequency are two important properties of any sound. Can we differentiate sounds on the basis of their amplitudes and frequencies? Loudness and Pitch Eardrum


Science

[Activity] ## Activity 10. 11 Take a metallic tumbler and a Human ear tablespoon. Strike the tablespoon gently at the brim of the tumbler. We must NEVER put a sharp, pointed or hard thing into our ear. It can damage the eardrum. The damaged eardrum can impair hearing. 10. 5 Amplitude, Time Period and Frequency of a Vibration We have learnt that the to and fro motion of an object is known as vibration. This Thermocol ball touching the motion is also called oscillatory motion. vibrating glass tumbler 2024-25 Compare the sound of a baby with Hear the sound produced. Now bang that of an adult. Is there any difference? the spoon on the tumbler and hear Even if two sounds are equally loud, the sound produced again. Is the they differ in some way. Let us see how. sound louder when the tumbler is struck hard? Now suspend a small thermocol I wonder why my voice ball touching the rim of the tumbler is different from that of (Fig. 10. 17). Vibrate the tumbler by my teacher. striking it. See how far the ball is displaced. The displacement of the The frequency determines the ball is a measure of the amplitude shrillness or pitch of a sound. If the of vibration of the tumbler. frequency of vibration is higher we say Now, strike the tumbler gently that the sound is shrill and has a higher and then a little harder. Compare pitch. If the frequency of vibration is the amplitudes of vibrations of the lower, we say that the sound has a lower tumbler in the two cases. In which pitch. For example, a drum vibrates with case is the amplitude larger? a low frequency. Therefore, it produces Loudness of sound is proportional to the square of the amplitude of the vibration producing the sound. For example, if the amplitude becomes twice, the loudness increases by a factor of 4. The loudness is expressed in a unit called decibel (dB). The following table gives some idea of the loudness of sound coming from various sources. Normal breathing 10 dB Soft whisper (at 5m) 30 dB Normal conversation 60 dB Busy traffic 70 dB Average factory 80 dB Above 80 dB the noise becomes physically painful. The loudness of sound depends on its amplitude. When the amplitude of vibration is large, the sound produced is loud. When the amplitude is small, Frequency determines the the sound produced is feeble. pitch of a sound 2024-25 a low-pitched sound. On the other 10. 7 Noise and Music hand, a whistle has a high frequency We hear different types of sounds and therefore, produces a sound of around us. Is the sound always higher pitch (Fig. 10. 18). A bird makes pleasing? Does a sound sometimes a high-pitched sound whereas a lion cause discomfort to you? Some sounds makes a low-pitched roar. However, the are pleasant to the ear, whereas some roar of a lion is very loud while the are not. sound of the bird is quite feeble. Suppose construction work is going Every day you hear the voices of on in your neighbourhood. Are the children and adults. Do you find any sounds coming from the construction difference in their voices? Can you say site pleasing? Do you enjoy the sounds that the frequency of the voice of a child produced by horns of buses and trucks? is higher than that of an adult? Such unpleasant sounds are called Usually the voice of a woman has noise. In a classroom, if all the students a higher frequency than that of a speak together, what would the sound man. produced be called? On the other hand you enjoy sounds 10. 6 Audible and Inaudible from musical instruments. Musical Sounds sound is one which is pleasing to the ear. Sound produced by a harmonium We know that we need a vibrating body is a musical sound. The string of a sitar for the production of sound. Can we also gives out a musical sound. But, if hear the sound of all vibrating bodies? a musical sound becomes too loud, The fact is that sounds of frequencies would it remain melodious? less than about 20 vibrations per second (20 Hz) cannot be detected by the human 10. 8 Noise Pollution ear. Such sounds are called inaudible. On the higher side, sounds of frequencies You already know about air pollution. higher than about 20, 00 vibrations per Presence of unwanted gases and particles in air is called air pollution. second (20 kHz) are also not audible to Similarly, presence of excessive or the human ear. Thus, for human ear, unwanted sounds in the environment the range of audible frequencies is is called noise pollution. Can you list roughly from 20 to 20, 00 Hz. some sources of noise pollution? Major causes of noise pollution are sounds of Some animals can hear sounds of frequencies higher than 20, 00 Hz. vehicles, explosions including bursting Dogs have this ability. The police use of crackers, machines, loudspeakers etc. high frequency whistles which dogs can What sources in the home may lead to hear but humans cannot. noise? Television and transistor radio The ultrasound equipment, familiar at high volumes, some kitchen to us for investigating and tracking appliances, desert coolers, air many medical problems, works at conditioners, all contribute to noise frequencies higher than 20, 00 Hz. pollution. 2024-25 What are the Harms of Noise must be installed in air craft engines, Pollution? transport vehicles, industrial machines and home appliances. Do you know that presence of excessive How can the noise pollution be noise in the surroundings may cause controlled in a residential area? many health related problems. Lack of All noisy operations must be sleep, hypertension (high blood- pressure), anxiety and many more health conducted away from any residential disorders may be caused by noise area. Noise producing industries pollution. A person who is exposed to a should be set up away from such loud sound continuously may get areas. Use of automobile horns temporary or even permanent should be minimised. TV and music impairment of hearing. systems should be run at low volumes. Trees must be planted along Measures to Limit Noise Pollution the roads and around buildings to To control noise, we must control the cut down on the sounds reaching the sources of noise. How can this be residents, thus reducing the harmful achieved? For this, silencing devices effects of noise pollution. Hearing Impairment Total hearing impairment, which is rare, is usually from birth itself. Partial disability is generally the result of a disease, injury or age. Children with impaired hearing need special care. By learning sign language, such children can communicate effectively. Because speech develops as the direct result of hearing, a child with a hearing loss may have defective speech also. Technological devices for the hearing-impaired have made it possible for such persons to improve their quality of life. Society can do much to improve the living environment for the hearing-impaired and help them live normal lives. 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT AMPLITUDE (cid: 220) Sound is produced by vibrating objects. (cid: 220) In human beings, the vibration of the vocal EARDRUM cords produces sound. FREQUENCY (cid: 220) Sound travels through


Science

[Exercise] ## Exercises 1. Choose the correct answer. Sound can travel through (a) gases only (b) solids only - liquids only (d) solids, liquids and gases. 2. Voice of which of the following is likely to have minimum frequency? (a) Baby girl (b) Baby boy - A man (d) A woman 2024-25 3. In the following statements, tick 'T' against those which are true, and 'F' against those which are false. (a) Sound cannot travel in vacuum. (T/F) (b) The number of oscillations per second of a vibrating object is called its time period. (T/F) - If the amplitude of vibration is large, sound is feeble. (T/F) - For human ears, the audible range is 20 Hz to 20, 00 Hz. (T/F) (e) The lower the frequency of vibration, the higher is the pitch. (T/F) (f) Unwanted or unpleasant sound is termed as music. (T/F) (g) Noise pollution may cause partial hearing impairment. (T/F) 4. Fill in the blanks with suitable words. (a) Time taken by an object to complete one oscillation is called __. (b) Loudness is determined by the __ of vibration. - The unit of frequency is __ - Unwanted sound is called __. (e) Shrillness of a sound is determined by the __ of vibration. 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency. 6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration? 7. Identify the part which vibrates to produce sound in the following instruments. (a) Dholak (b) Sitar (c) Flute 8. What is the difference between noise and music? Can music become noise sometimes? 9. List sources of noise pollution in your surroundings. 10. Explain in what way noise pollution is harmful to human. 11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer. 12. Sketch larynx and explain its function in your own words. 13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why? S E S I C R E X E 2024-25 Extended Learning - ## Activities and Projects 1. Visit the music room of your school. You may also visit musicians in your locality. Make a list of musical instruments. Note down the parts of these instruments that vibrate to produce sound. 2. If you play a musical instrument, bring it to the class and demonstrate how you play it. 3. Prepare a list of famous Indian musicians and the instruments they play. 4. Take a long thread. Place your hands over your ears and get some one to place this thread round your head and hands. Ask her to make the thread taut and hold its ends in one hand. Now ask her to draw her finger and thumb tightly along the thread (Fig. 10. 19). Can you hear a rolling sound like that of a thunder? Now repeat the activity while another friend stands near both of you. Can he hear any sound? Fig. 10. 19 5. Make two toy telephones. Use them as shown in Fig. 10. 20. Make sure that the two strings are taut and touch each other. Let one of you speak. Can the remaining three persons hear? See how many more friends you can engage in this way. Explain your observations. Fig. 10. 20 2024-25 6. Identify the sources of noise pollution in your locality. Discuss with your parents, friends and neighbours. Suggest how to control noise pollution. Prepare a brief report and present it in the class. You can read more on the related topics on the following websites: - ww. physicsclassroom. com/Class/sound/soundtoc. html - health. howstuffworks. com/hearing. htm Did You Know? Golconda fort, near Hyderabad, is one of the most magnificient forts in India. It is famous for many engineering and architectural marvels. One of the marvels is the water supply system. But, perhaps, more astonishing is a dome near the entrance to the fort. A hand-clap at a particular point under the dome reverberates and can be heard at the highest point of the fort, about a kilometre away. This was devised as a warning system. If a guard saw a suspicious movement outside the fort, he clapped at the particular point under the dome, and the army inside the fort was alerted to the danger of the approaching enemy. Golconda fort 2024-25


Science

CC EE HHEEMMIICCAALL FFEECCTTSS OOFF EE CC LLEECCTTRRIICC UURREENNTT Y our elders might have cautioned plastic and wood do not conduct you against touching an electrical electricity. However, so far we have used appliance with wet hands. But our tester to test materials which were do you know why it is dangerous to in solid state. But what about liquids? touch an electrical appliance with wet Do liquids also conduct electricity? Let hands? us find out. We have learnt earlier that the materials, which allow electric current to pass through them, are good Paheli and Boojho want to conductors of electricity. On the other remind you that one hand, materials, which do not allow should not experiment electric current to pass through them with the electric supply easily, are poor conductors of electricity. from the mains or a In Class VI, we made a tester to test generator or an inverter. Use only electric cells for whether a particular material allows the all the activities suggested electric current to pass through it or not. here. Do you recall how the tester helped us in deciding that? 11. 1 Do Liquids Conduct We found that metals such as copper and aluminium conduct electricity Electricity? whereas materials such as rubber, To test whether a liquid allows electric current to pass through it or not, we can use the same tester (Fig. 11. 1). A tester 2024-25 However, replace the cell by a battery. of the tester glow? Does lemon juice Also, before using the tester we or vinegar conduct electricity? How should check whether it is working would you classify lemon juice or or not. vinegar- a good conductor or a poor conductor?


Science

[Activity] ## Activity 11. 1 Join the free ends of the tester together for a moment. This completes the circuit of the tester and the bulb should glow. However, if the bulb does not glow, it means that the tester is not working. Can you think of the possible reasons? Is it possible that the connections are loose? Or, the bulb is fused? Or, your cells are used up? Check that all the connections are tight. If they are, then replace the bulb with another bulb. Now test if the tester is working or not. If it is still not Testing conduction of electricity in lemon juice or vinegar working then replace the cells with fresh cells. When the liquid between the two ends of the tester allows the electric Now that our tester is working, let current to pass, the circuit of the tester us use it to test the various liquids. becomes complete. The current flows in (Caution: While checking your tester, the circuit and the bulb glows. When do not join its free ends for more than a the liquid does not allow the electric few seconds. Otherwise the cells of the current to pass, the circuit of the tester battery will drain very quickly. ) is not complete and the bulb does not glow.


Science

[Activity] ## Activity 11. 2 In some situations even though the Collect a few small plastic or rubber liquid is conducting, the bulb may not caps of discarded bottles and clean glow. It may have happened in them. Pour one teaspoon of lemon ## Activity 11. 2. What can be the reason? juice or vinegar in one cap. Bring Do you remember why the bulb your tester over this cap and let the glows when the electric current passes through it? Due to the heating effect ends of the tester dip into lemon juice of current, the filament of the bulb gets or vinegar as shown in Fig. 11. 2. Take heated to a high temperature and it care that the ends are not more than starts glowing. However, if the current 1 cm apart but at the same time do through a circuit is too weak, the not touch each other. Does the bulb filament does not get heated 2024-25 sufficiently and it does not glow. And ## Activity 11. 3 why is the current in the circuit weak? Take the tray from inside a discarded Well, though a material may conduct matchbox. Wrap an electric wire a few electricity, it may not conduct it as times around the tray. Place a small easily as a metal. As a result, the compass needle inside it. Now circuit of the tester may be complete connect one free end of the wire to and yet the current through it may be the terminal of a battery. Leave the too weak to make the bulb glow. Can other end free. Take another piece of we make another tester which can wire and connect it to the other detect a weak current? terminal of the battery (Fig. 11. 4). We can use another effect of an electric current to make another kind of tester. Do you recall that electric current produces a magnetic effect? What happens to a compass needle kept nearby when current flows in a wire? Even if the current is small, the deflection of the magnetic needle can be seen. Can we make a tester using the magnetic effect of currents? Let us find out in


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[Activity] ## Activity 11. 3. You may use an LED (Fig. 11. 3) in Fig 11. 4: Another tester place of the electric bulb in the tester of Fig. 11. 2. LED glows even when a Join the free ends of two wires weak electric current flows through momentarily. The compass needle it. should show deflection. Your tester There are two wires (called leads) with two free ends of the wire is ready. attached to an LED. One lead is Now repeat ## Activity 11. 2 using this slightly longer than the other. tester. Do you find a deflection in the


Science

## Remember that while connecting to compass needle the moment you dip a circuit, the longer lead is always the free ends of the tester in lemon connected to the positive terminal of juice? the battery and the shorter lead is Take out the ends of the tester connected to the negative terminal of from the lemon juice, dip them in the battery. water and then wipe them dry. Repeat the activity with other liquids such as tap water, vegetable oil, milk, honey. (## Remember to wash and wipe dry the ends of tester after testing each liquid). In each case observe whether the magnetic needle shows deflection or not. Record your LEDs observations in Table 11. 1. 2024-25 Good/Poor Conducting Liquids S. No. Material Compass Needle Shows Good Conductor/ Deflection Yes/No Poor Conductor 1. Lemon juice Yes Good Conductor 2. Vinegar 3. Tap Water 4. Vegetable oil 5. Milk 6. Honey 7. 8. 9. 10. From Table 11. 1, we find that some We have tested the conduction of liquids are good conductors of electricity electricity through tap water. Let us now and some are poor conductors. test the conduction of electricity through distilled water.


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[Activity] ## Activity 11. 4 When the free ends of the tester do not touch each Take about two teaspoonfuls of other, there is an air gap distilled water in a clean and dry between them. Paheli knows plastic or rubber cap of a bottle. (You that air is a poor conductor of may obtain distilled water from your electricity. But she has also read school science lab. You may also get that during lightning, an electric distilled water from a medical store current passes through air. She or a doctor or a nurse). Use the tester wonders if air is indeed a poor conductor under all conditions. to test whether distilled water This makes Boojho ask whether conducts electricity or not. What do other materials classified as you find? Does distilled water poor conductors also allow conduct electricity? Now dissolve a electricity to pass under pinch of common salt in distilled certain conditions. water. Again test. What do you conclude this time? Actually, under certain conditions When salt is dissolved in distilled most materials can conduct. That is why water, we obtain salt solution. This is a it is preferable to classify materials as conductor of electricity. good conductors and poor conductors The water that we get from sources instead of classifying as conductors and such as taps, hand pumps, wells and insulators. ponds is not pure. It may contain 2024-25 several salts dissolved in it. Small Most liquids that conduct electricity amounts of mineral salts are naturally are solutions of acids, bases and salts. present in it. This water is thus a good When an electric current flows conductor of electricity. On the other through a conducting solution, does it hand, distilled water is free of salts and produce an effect on the solution? is a poor conductor. 11. 2 Chemical Effects of Electric Current Small amounts of mineral salts In Class VII, we have learnt some effects present naturally in water are of electric current. Can you list these beneficial for human health. However, these salts make water effects? What effect does the current a good conductor. So, we should produce when it flows through a never handle electrical conducting solution? Let us find out. appliances with wet hands or while standing on a wet floor. ## Activity 11. 6 We have found that common salt, Take out carbon rods carefully from when dissolved in distilled water, makes two discarded cells. Clean their it a good conductor. What are the other metal caps with sand paper. Wrap substances which, when dissolved in copper wires around the metal caps distilled water, make it conducting? Let of the carbon rods and join them to us find out. a battery (Fig. 11. 5). We call these Caution: Do the next activity under the two rods electrodes. (Instead of supervision of your teacher/parent or some elderly person, because the use of acid is involved in it.


Science

[Activity] ## Activity 11. 5 Take three clean plastic or rubber caps of bottles. Pour about two teaspoonfuls of distilled water in each of them. Add a few drops of lemon juice or dilute hydrochloric acid to distilled water in one cap. Now in the second cap containing distilled water, add a few Metal Cap drops of a base such as caustic soda or potassium iodide. Add a little sugar Carbon Carbon to the distilled water in the third cap rod rod and dissolve it. Test which solutions Water conduct electricity and which do not. What results do you obtain? Passing current through water 2024-25 carbon rods, you may take two iron nails about 6 cm long. ) Pour a cupful of water in a glass/plastic Boojho decided to test whether some fruits and bowl. Add a teaspoonful of salt or a vegetables also conduct few drops of lemon juice to water to electricity or not. He cut make it more conducting. Now a potato into two halves and immerse the electrodes in this inserted the copper wires of a tester solution. Make sure that the metal into it. Just then his mother called caps of the carbon rods are outside him and he forgot to take out the the water. Wait for 3-4 minutes. wires of the tester inserted into the Observe the electrodes carefully. Do potato. When he came back after half an hour, he noticed that there you notice any gas bubbles near the was a greenish blue spot on the electrodes? Can we call the change potato around one wire whereas taking place in the solution a there was no such spot around the chemical change? Recall the other wire (Fig. 11. 6). definition of a chemical change that you learnt in Class VII. In 1800, a British chemist, William Nicholson (1753-1815), had shown that if electrodes were immersed in water, and a current was passed, bubbles of oxygen and hydrogen were produced. Oxygen bubbles formed on the electrode Testing potato connected to the positive terminal of He was surprised with this the battery and hydrogen bubbles observation and along with Paheli formed on the other electrode. repeated this activity many times. They The passage of an electric current found that it was always the wire connected to the positive terminal, through a conducting solution causes which had a greenish blue spot around chemical reactions. As a result, bubbles it. They felt that this discovery was very of a gas may be formed on the electrodes. useful because it could be used for Deposits of metal may be seen on identifying the positive terminal of a electrodes. Changes of colour of cell or a battery concealed in a box. solutions may occur. The reaction would They decided to report their finding to depend on what solution and electrodes a children's magazine. are used. These are some of the chemical ## Remember that Boojho set out to test whether potato conducted effects of the electric current. 2024-25 electricity or not. What he found was that current produced a chemical effect in the potato. To him this was very exciting. In fact, this is how science sometimes works. You are looking for something and you discover something else. Many Copper plate important discoveries have been made in this manner. Copper plate Copper sul- 11. 3 Electroplating phate solution You know that a brand new bicycle has shiny handlebar and wheel rims. A simple circuit showing electroplating However, if these are accidentally scratched, the shiny coating comes off Allow the current to pass for revealing a not so shiny surface beneath. about 15 minutes. Now remove the You might have also seen women using electrodes from the solution and ornaments, which appear to be made of look at them carefully. Do you find gold. However, with repeated use, the any difference in any one of them? gold coating wears off, revealing silver or Do you find a coating over it? What some other metal beneath. colour is the coating? Note down the In both these cases, a metal has a terminal of the battery with which coating of another metal. Do you wonder this electrode is connected. how a layer of one metal can be deposited on top of another? Well, let us try doing it ourselves. After doing the electroplating activity, Paheli interchanged


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[Activity] ## Activity 11. 7 the electrodes and repeated the activity. What do you think We will need copper sulphate and she would observe this time? two copper plates of size around 10 cm × 4 cm. Take 250 mL of distilled water in a clean and dry When electric current is passed beaker. Dissolve two teaspoonfuls of through the copper sulphate solution, copper sulphate in it. Add a few drops copper sulphate dissociates into of dilute sulphuric acid to copper copper and sulphate. The free copper sulphate solution to make it more gets drawn to the electrode connected to the negative terminal of the battery conducting. Clean copper plates with and gets deposited on it. But what sand paper. Now rinse them with about the loss of copper from the water and dry them. Connect the solution? copper plates to the terminals of a From the other electrode, a copper battery and immerse them in copper plate, an equal amount of copper gets sulphate solution (Fig. 11. 7). dissolved in the solution. Thus, the 2024-25 loss of copper from the solution is restored and the process continues. This means that copper gets transferred from one electrode to the other. Boojho could get only one copper plate. So he performed ## Activity 11. 7 by connecting a carbon rod in place of the copper plate which was connected to the negative terminal of the battery. He succeeded in obtaining a coating of copper on carbon rod. The process of depositing a layer of any desired metal on another material by means of electricity is called Some electroplated objects electroplating. It is one of the most silver and gold on less expensive metals. common applications of chemical effects These ornaments have the appearance of electric current. of silver or gold but are much less Electroplating is a very useful expensive. process. It is widely used in industry Tin cans, used for storing food, are for coating metal objects with a thin made by electroplating tin onto iron. Tin layer of a different metal (Fig. 11. 8). The is less reactive than iron. Thus, food layer of metal deposited has some does not come into contact with iron and desired property, which the metal of the is protected from getting spoilt. object lacks. For example, chromium Iron is used in bridges and plating is done on many objects such automobiles to provide strength. as car parts, bath taps, kitchen gas However, iron tends to corrode and rust. burners, bicycle handlebars, wheel rims So, a coating of zinc is deposited on iron and many others. to protect it from corrosion and Chromium has a shiny appearance. formation of rust. It does not corrode. It resists scratches. However, chromium is expensive and it In the electroplating factories the may not be economical to make the disposal of the used conducting whole object out of chromium. So the solution is a major concern. It is a object is made from a cheaper metal and polluting waste and there are specific disposal guidelines to protect the only a coating of chromium over it is environment. deposited. Jewellery makers electroplate 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT ELECTRODE (cid: 220) Some liquids are good conductors of electricity and some are poor conductors. ELECTROPLATING (cid: 220) Most liquids that conduct electricity are GOOD CONDUCTOR solutions of acids, bases and salts. (cid: 220) The passage of an electric current through a LED conducting liquid causes chemical reactions. POOR CONDUCTOR The resulting effects are called chemical effects of currents. (cid: 220) The process of depositing a layer of any desired metal on another material, by means of electricity, is called electroplating.


Science

[Exercise] ## Exercises 1. Fill in the blanks. (a) Most liquids that conduct electricity are solutions of, __ and __. (b) The passage of an electric current through a solution causes __ effects. - If you pass current through copper sulphate solution, copper gets deposited on the plate connected to the terminal of the battery. - The process of depositing a layer of any desired metal on another material by means of electricity is called. 2. When the free ends of a tester are dipped into a solution, the magnetic needle shows deflection. Can you explain the reason? 3. Name three liquids, which when tested in the manner shown in Fig. 11. 9, may cause the magnetic needle to deflect. Fig. 11. 9 2024-25 4. The bulb does not glow in the setup shown in Fig. 11. 10. List the possible reasons. Explain your answer. Fig. 11. 10 5. A tester is used to check the conduction of electricity through two liquids, labelled A and B. It is found that the bulb of the tester glows brightly for liquid A while it glows very dimly for liquid B. You would conclude that - liquid A is a better conductor than liquid B. - liquid B is a better conductor than liquid A. - both liquids are equally conducting. - conducting properties of liquid cannot be compared in this manner. 6. Does pure water conduct electricity? If not, what can we do to make it conducting? 7. In case of a fire, before the firemen use the water hoses, they shut off the main electrical supply for the area. Explain why they do this. 8. A child staying in a coastal region tests the drinking water and also the seawater with his tester. He finds that the compass needle deflects more in the case of seawater. Can you explain the reason? 9. Is it safe for the electrician to carry out electrical repairs outdoors during heavy downpour? Explain. 10. Paheli had heard that rainwater is as good as distilled water. So she collected some rainwater in a clean glass tumbler and tested it using a tester. To her surprise she found that the compass needle showed deflection. What could be the reasons? 11. Prepare a list of objects around you that are electroplated. 12. The process that you saw in ## Activity 11. 7 is used for purification of copper. A thin plate of pure copper and a thick rod of impure copper are used as electrodes. Copper from impure rod is sought to be transfered to the thin copper plate. Which electrode should be attached to the positive terminal of the battery and why? S E S I C R E X E 2024-25 Extended Learning - ## Activities and Projects 1. Test the conduction of electricity through various fruits and vegetables. Display your result in a tabular form. 2. Repeat ## Activity 11. 7 with a zinc plate in place of the copper plate connected to the negative terminal of the battery. Now replace zinc plate with some other metallic object and again repeat the activity. Which metal gets deposited over which other metal? Discuss your findings with your friends. 3. Find out if there is a commercial electroplating unit in your town. What objects are electroplated there and for what purpose? (The process of electroplating in a commercial unit is much more complex than what we did in ## Activity 11. 7). Find out how they dispose off the chemicals they discard. 4. Imagine that you are an 'entrepreneur' and have been provided a loan by a bank to set up a small electroplating unit. What object would you like to electroplate and for what purpose? (Look up the meaning of 'entrepreneur' in a dictionary). 5. Find out the health concerns associated with chromium electroplating. How are people trying to resolve them? 6. You can make a fun pen for yourself. Take a conducting metal plate and spread a moist paste of potassium iodide and starch. Connect the plate to a battery as shown in Fig. 11. 11. Now using the free end of the wire, write a few letters on the paste. What do you see? Fig. 11. 11 2024-25 For more information on this topic visit: - electronics. howstuffworks. com/led. htm Did You Know? LEDs (Light Emitting Diodes) are available in many colours such as red, green, yellow, blue, white and are increasingly being used for many applications, for example in traffic signal lights. LEDs are increasingly being used for lighting. A cluster of white LEDs grouped together forms a LED light source. LED light sources consume less electricity and have longer lifetime than light bulbs and fluorescent tubes. Hence these are gradually becoming the preferred lighting source. 2024-25


Science

SS NN PP OOMMEE AATTUURRAALL HHEENNOOMMEENNAA In this chapter we shall discuss two kind of resin) was rubbed with fur, it destructive natural phenomena. These attracted light objects such as hair. You might have seen that when you take are lightning and earthquakes. We shall off woollen or polyester clothes, your also discuss what steps we can take to hair stands on end. If you take off these minimise destruction caused by clothes in the dark, you even see a these phenomena. spark and hear a crackling sound. In 1752 Benjamin Franklin, an American 12. 1Lightning scientist, showed that lightning and the You might have seen sparks on a spark from your clothes are essentially electric pole when wires become loose. the same phenomena. However, it took This phenomenon is quite common 200 years for this realisation to occur. when wind is blowing and shaking the wires. You might also have seen sparks when a plug is loose in its socket. I wonder why they took so Lightning is also an electric spark, but many years to realise the on a huge scale. similarity. In ancient times, people did not understand the cause of these sparks. They were, therefore, afraid of lightning and thought that the wrath of gods was Scientific discoveries are a visiting them. Now, of course, we result of hardwork by many understand that lightning is caused by people. It can sometimes take the accumulation of charges in the a long time. clouds. We need not be afraid of lightning, but we have to take We shall now study some properties precautions to protect ourselves from of electric charges. We shall also see how the deadly sparks. they are related to the lightning in the sky. The Sparks that the Greeks Knew Let us perform some activities to About understand the nature of electric The ancient Greeks knew as early as charges. But recall first what you might 600 B. C. that when amber (amber is a have played as a game. When you rub 2024-25 a plastic scale on your dry hair, the small pieces of dry leaf, husk and scale can attract very small pieces of mustard seeds. Record your paper. observations. 12. 2 Charging by Rubbing When a plastic refill is rubbed with


Science

[Activity] ## Activity 12. 1 polythene, it acquires a small electric charge. Similarly, when a plastic comb Take a used ballpen refill and rub is rubbed with dry hair, it acquires a it vigorously with a piece of small charge. These objects are called polythene. Bring it close to small charged objects. In the process of pieces of paper. Take care not to charging the refill and the plastic comb, touch the rubbed end of the refill polythene and hair also get charged. with your hand or with a metallic object. Repeat the activity with Let's try to charge some other objects that are familiar to you.


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[Activity] ## Activity 12. 2 Collect the objects and the materials listed in Table 12. 1. Try to charge each by rubbing with the materials mentioned in the Table. Record your findings. You can add more items to the Table. Table 12. 1 Objects Rubbed Materials Attracts/does not Charged/ Used for Rubbing Attract Pieces Not Charged of Paper Refill Polythene, woollen cloth Balloon Polythene, woollen cloth, dry hair Eraser Wool Steel spoon Polythene, woollen cloth 2024-25


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12.3 Types of Charges and Rub the other refill also with Their Interaction polythene. Bring it close to the charged

12. 3 Types of Charges and Rub the other refill also with Their Interaction polythene. Bring it close to the charged refill. Be careful not to We will select some objects from touch the charged end with your Table 12. 1 for the next activity. hand. Is there any effect on the refill


Science

Activity 12.3 in the tumbler? Do the two attract

[Activity] ## Activity 12. 3 in the tumbler? Do the two attract each other, or repel each other? (a) Inflate two balloons. Hang them In this activity we have brought in such a way that they do not touch close together the charged objects each other (Fig. 15. 1). Rub both the that were made of the same balloons with a woollen cloth and material. What happens if two release them. What do you observe? charged objects made of different materials are brought close to each other? Let's find out. (b) Rub a refill and place it gently in a glass tumbler as before (Fig. 12. 3). Bring an inflated charged balloon near the refill and observe. Like charges repel each other Now let us repeat this activity with the used pen refills. Rub one refill with polythene. Place it carefully in a glass tumbler using the tumbler as a stand (Fig. 12. 2). Unlike charges attract each other Let's summarise the observations: - A charged balloon repelled a charged balloon. - A charged refill repelled a charged refill. - But a charged balloon attracted a charged refill. Does it indicate that the charge on the balloon is of a different kind from the charge on the refill? Can we say Interaction between like charges then, that there are two kinds of 2024-25 charges? Can we also say that the it with the end of the paper clip. charges of the same kind repel each Observe what happens. Is there any other, while charges of different kind effect on the foil strips? Do they attract each other? repel each other or attract each It is a convention to call the charge other? Now, touch other charged acquired by a glass rod when it is bodies with the end of the paper clip. rubbed with silk as positive. The Do foil strips behave in the same other kind of charge is said to be way in all cases? Can this apparatus negative. be used to detect whether a body is It is observed that when a charged charged or not? Can you explain glass rod is brought near a charged why the foil strips repel each other? plastic straw rubbed with polythene there is attraction between the two. What do you think would be the kind of charge on the plastic straw? Your guess, that the plastic straw would carry a negative charge is correct. The electrical charges generated by rubbing are static. They do not move by themselves. When charges move, they constitute an electric current. You have been reading about electric current since Class VI. The current in a circuit which makes a bulb glow, or the current that makes a wire hot, is nothing but a motion of charges. Fig 12. 4: A simple electroscope 12. 4Transfer of Charge The aluminium foil strips receive the


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[Activity] ## Activity 12. 4 same charge from the charged refill through the paper clip (remember that Take an empty jam bottle. Take a metals are good conductors of piece of cardboard slightly bigger in electricity). The strips carrying similar size than the mouth of the bottle. charges repel each other and they Pierce a hole in it so that a metal become wide open. Such a device can paper clip can be inserted. Open out be used to test whether an object is the paper clip as shown in Fig. 12. 4. carrying charge or not. This device is Cut two strips of aluminium foil known as electroscope. about 4 cm × 1 cm each. Hang them Thus, we find that electrical charge on the paper clip as shown. Insert can be transferred from a charged object the paper clip in the cardboard lid to another through a metal conductor. so that it is perpendicular to it Touch the end of the paper clip gently (Fig. 12. 4). Charge a refill and touch with hand and you will find a change in 2024-25 the foil strips. They come back to their During the development of a original state. Repeat charging of foil thunderstorm, the air currents move strips and touching the paper clip. Every upward while the water droplets move time you will find that the foil strips downward. These vigorous movements collapse as soon as you touch the cause separation of charges. By a paperclip with hand. Why does it process, not yet completely understood, happen? The reason is that the foil strips the positive charges collect near the lose charge to the earth through your upper edges of the clouds and the body. We say that the foil strips are negative charges accumulate near the discharged. The process of transferring lower edges. There is accumulation of of charge from a charged object to the positive charges near the ground also. earth is called earthing. When the magnitude of the accumulated charges becomes very Earthing is provided in buildings to large, the air which is normally a poor protect us from electrical shocks due to any leakage of electrical current. conductor of electricity, is no longer able to resist their flow. Negative and positive 12. 5 The Story of Lightning charges meet, producing streaks of bright light and sound. We see streaks It is now possible to explain lightning as lightning (Fig. 12. 5). The process is in terms of the charges produced by called an electric discharge. rubbing. Accumulation of charges leading to lightning. 2024-25 The process of electric discharge can metal objects. Do not lie on the ground. occur between two or more clouds, or Instead, squat low on the ground. Place between clouds and the earth. Today we your hands on your knees with your need not get frightened by lightning like head between the hands (Fig. 12. 6). This our ancestors did. Now we understand position will make you the smallest the basic phenomenon. Scientists are target to be struck. trying hard to improve this understanding. However, lightning strike could destroy life and property. It is, therefore, necessary to take measures to protect ourselves. 12. 6 Lightning Safety During lightning and thunderstorm no open place is safe. - Hearing thunder is an alert to rush to a safer place. - After hearing the last thunder, wait for some time before coming out of the safe place. Finding a Safe Place A house or a building is a safe place. If you are travelling by car or by bus, Safe position during lightning you are safe inside with windows and doors of the vehicle shut. Inside the house Lightning can strike telephone cords, Do's and Don'ts during a electrical wires and metal pipes (Do you Thunderstorm remember, lightning is an electrical Outside discharge? ). During a thunderstorm Open vehicles, like motorbikes, tractors, contact with these should be avoided. It construction machinery, open cars are is safer to use mobile phones and not safe. Open fields, tall trees, shelters cordless phones. However, it is not wise in parks, elevated places do not protect to call up a person who is receiving your us from lightning strokes. phone through a wired phone. Carrying an umbrella is not at all a Bathing should be avoided during good idea during thunderstorms. thunderstorms to avoid contact with If in a forest, take shelter under running water. shorter trees. Electrical appliances like computers, If no shelter is available and you are TVs, etc. , should be unplugged. Electrical in an open field, stay far away from all lights can remain on. They do not cause trees. Stay away from poles or other any harm. 2024-25 Lightning Conductors 12. 7Earthquakes Lightning Conductor is a device used You just learnt about thunderstorm and to protect buildings from the effect of lightning. These natural phenomena can lightning. A metallic rod, taller than the cause large scale destruction of human building, is installed in the walls of the life and property. Fortunately, these building during its construction. One phenomena can be predicted to some end of the rod is kept out in the air and extent. The weather department can the other is buried deep in the ground warn about a thunderstorm developing (Fig. 12. 7). The rod provides easy route in some area. for the transfer of electric charge to the If a thunderstorm occurs there is ground. always a possibility of lightning and The metal columns used during cyclones accompanying it. So, we get construction, electrical wires and water time to take measures to protect pipes in the buildings also protect us to ourselves from the damage caused by these phenomena. an extent. But do not touch them during a thunderstorm. lightning conductor copper plate Lightning conductor 2024-25 There is, however, one natural caused by these earthquakes from phenomenon which we are not yet able newspapers and magazines of those to predict accurately. It is an days. Prepare a short report on the earthquake. It can cause damage to suffering of the people during and human life and property on a huge scale. after the earthquakes. A major earthquake occurred in India on 8 October 2005 in Uri and What is an earthquake? What Tangdhar towns of North Kashmir happens when it occurs? What can we (Fig. 12. 8). Before that a major do to minimise its effects? These are earthquake occurred on 26 January some of the questions which we shall 2001 in Bhuj district of Gujarat. discuss below.


Science

[Activity] ## Activity 12. 5 What is an Earthquake? Ask your parents about the huge An earthquake is a sudden shaking or damage to life and property caused trembling of the earth which lasts for a by these earthquakes. Collect a few very short time. It is caused by a pictures showing the damage disturbance deep inside the earth's Kashmir earthquake 2024-25 crust. Earthquakes occur all the time, all over the earth. They are not even What could cause a noticed. Major earthquakes are much disturbance inside less frequent. They can cause immense the earth? damage to buildings, bridges, dams and Now we know that the tremors are people. There can be a great loss to life caused by the disturbance deep down and property. Earthquakes can cause inside the uppermost layer of the earth floods, landslides and tsunamis. A called the crust (Fig. 12. 9). major tsunami occurred in the Indian Ocean on 26 December 2004. All the coastal areas around the ocean suffered huge losses. crust


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[Activity] ## Activity 12. 6 mantle Take an outline map of the world. Locate the eastern coast and outer core Andaman and Nicobar Islands in India. Mark other countries around inner the Indian Ocean which could have core suffered damage. Collect accounts of the devastation caused by the tsunami in India from your parents, Structure of the earth or other elders in the family or in The outermost layer of the earth is the neighbourhood. not in one piece. It is fragmented. Each fragment is called a plate (Fig. 12. 10). What Causes an Earthquake? These plates are in continual motion. When they brush past one another, My grandmother told me that the earth is balanced on the horn of a bull and when the bull shifts it to the other horn, an earthquake takes place. How could it be true? Eurasian In ancient times, people did not know Plate the true cause of earthquakes. Their Arabian ideas were, therefore, expressed in Plate Indan mythical stories such as the one told by Plate Boojho's grandmother. Similar myths were prevalent in other parts of the world. Earth plates 2024-25 or a plate goes under another due to carried out. However, most earthquakes collision (Fig. 12. 11), they cause are caused by the movement of earth's disturbance in the earth's crust. It is plates. this disturbance that shows up as an Since earthquakes are caused by the earthquake on the surface of the earth. movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones. In India, the Colliding areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo-Gangetic Plane. Some areas of South India also fall in Brushing the danger zone (Fig. 12. 12). past Movements of earth's plates If scientists know so much about earthquakes, can they also predict when and where the next one will strike? Although, we know for sure what causes an earthquake, it is not yet possible to predict when and where the next earthquake might occur. Movements of Indian earth's plate I read somewhere that The power of an earthquake is underground explosions expressed in terms of a magnitude could also cause on a scale called the Richter scale. tremors. Really destructive earthquakes have Tremors on the earth can also be magnitudes higher than 7 on the caused when a volcano erupts, or a Richter scale. Both Bhuj and Kashmir meteor hits the earth, or an earthquakes had magnitudes greater underground nuclear explosion is than 7. 5. 2024-25 The tremors produce waves on the surface of the earth. These are called seismic waves. The waves are recorded by an instrument called the seismograph (Fig. 12. 13). The instrument is simply a vibrating rod, or a pendulum, which starts vibrating when tremors occur. A pen is attached to the vibrating system. The pen records the seismic waves on a paper which moves under it. By studying these waves, scientists can construct a complete map of the earthquake, as shown in Fig. 12. 14. They can also estimate its power to cause destruction. String Earth's Focus Surface Seismic waves Support Pendulum Bob Epicentre Pen Rotating Drum Chart Paper Inside the earth A typical seismograph record A seismograph Map of the earthquake Like many other scales in science (decibel is another example), Richter scale is not linear. This means that an earthquake of magnitude 6 does not have one and half times the destructive energy of an earthquake of magnitude 4. In fact, an increase of 2 in magnitude means 100 times more destructive energy. Therefore, an earthquake of magnitude 6 has thousand times more destructive energy than an earthquake of magnitude 4. Protection against Earthquakes living in seismic zones, where the earthquakes are more likely to occur, We know from the earlier pages that have to be specially prepared. First of earthquakes cannot be predicted. We have also seen that they can be highly all, the buildings in these zones should destructive. It is, therefore, important be so designed that they can withstand that we take necessary precautions to major tremors. Modern building protect ourselves all the time. People technology can make it possible. 2024-25 It is advisable to make the structure The Central Building Research simple so that it is 'Quake Safe'. Institute, Roorkee, has developed knowhow to make quake-resistant - Consult qualified architects and houses. structural engineers. In the event that an earthquake does - In highly seismic areas, the use of strike, take the following steps to protect mud or timber is better than using yourself. heavy construction material. Keep roofs as light as possible. In case 1. If you are at home the structure falls, the damage will l Take shelter under a table and stay not be heavy. there till the shaking stops. - It is better if the cupboards and l Stay away from tall and heavy objects shelves are fixed to the walls, so that that may fall on you. they do not fall easily. - If you are in bed, do not get up. - Be careful where you hang wall Protect your head with a pillow. clocks, photo-frames, water heaters 2. If you are outdoors etc. , so that in the event of an earthquake, they do not fall on l Find a clear spot, away from people. buildings, trees and overhead power - Since some buildings may catch fire lines. Drop to the ground. due to an earthquake, it is necessary l If you are in a car or a bus, do not that all buildings, especially tall come out. Ask the driver to drive buildings, have fire fighting slowly to a clear spot. Do not come equipment in working order. out till the tremors stop. 2024-25 KKEEYYWWOORRDDSS WWHHAATT YYOOUU HHAAVVEE LLEEAARRNNTT CRUST (cid: 220) Some objects can be charged by rubbing with other objects. DISCHARGE (cid: 220) There are two kinds of charges - positive EARTH'SPLATES charge and negative charge EARTHQUAKE (cid: 220) Like charges repel and unlike charges attract each other. ELECTROSCOPE (cid: 220) The electrical charges produced by rubbing LIGHTNING are called static charges. (cid: 220) When charges move, they constitute an LIGHTNING electric current. CONDUCTOR (cid: 220) An electroscope may be used to detect NEGATIVE CHARGE whether a body is charged or not. (cid: 220) The process of transfer of charge from a POSITIVE CHARGE charged object to the earth is called earthing. (cid: 220) The process of electric discharge between RICHTER SCALE clouds and the earth or between different SEISMOGRAPH clouds causes lightning. (cid: 220) Lightning strike could destroy life and THUNDER property. THUNDERSTORM (cid: 220) Lightning conductors can protect buildings from the effects of lightning. TRANSFER OF (cid: 220) An earthquake is a sudden shaking or CHARGE trembling of the earth. TSUNAMI (cid: 220) Earthquake is caused by a disturbance deep inside the earth's crust. TREMOR (cid: 220) It is not possible to predict the occurrence of an earthquake. (cid: 220) Earthquakes tend to occur at the boundaries of earth's plates. These boundaries are known as fault zones. (cid: 220) Destructive energy of an earthquake is measured on the Richter scale. The earthquake measuring


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[Exercise] ## Exercises Select the correct option in Questions 1 and 2. 1. Which of the following cannot be charged easily by friction? (a) A plastic scale (b) A copper rod - An inflated balloon - A woollen cloth. 2. When a glass rod is rubbed with a piece of silk cloth the rod (a) and the cloth both acquire positive charge. (b) becomes positively charged while the cloth has a negative charge. - and the cloth both acquire negative charge. - becomes negatively charged while the cloth has a positive charge. 3. Write T against true and F against false in the following statements. (a) Like charges attract each other (T/F) (b) A charged glass rod attract a charged plastic straw (T/F) - Lightning conductor cannot protect a building from lightning (T/F) - Earthquakes can be predicted in advance (T/F) 4. Sometimes, a crackling sound is heard while taking off a sweater during winters. Explain. 5. Explain why a charged body loses its charge if we touch it with our hand. 6. Name the scale on which the destructive energy of an earthquake is measured. An earthquake measures 3 on this scale. Would it be recorded by a seismograph? Is it likely to cause much damage? 7. Suggest three measures to protect ourselves from lightning. 8. Explain why a charged balloon is repelled by another charged balloon whereas an uncharged balloon is attracted by another charged balloon? 9. Describe with the help of a diagram an instrument which can be used to detect a charged body. 10. List three states in India where earthquakes are more likely to strike. 11. Suppose you are outside your home and an earthquake strikes. What precaution would you take to protect yourself? 12. The weather department has predicted that a thunderstorm is likely to occur on a certain day. Suppose you have to go out on that day. Would you carry an umbrella? Explain. 2024-25 Extended Learning - ## Activities and Projects 1. Open a water tap. Adjust the flow so that it forms a thin stream. Charge a refill. Bring it near the water stream. Observe what happens. Write a short report on the activity. 2. Make your own charge detector. Take a paper strip roughly 10 cm ×


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3 cm. Give it a shape as shown in Fig. 12.15. Balance it on a needle. Bring a charged body near it. Observe what

3 cm. Give it a shape as shown in Fig. 12. 15. Balance it on a needle. Bring a charged body near it. Observe what happens. Write a brief report, explaining its working. Paper 10 cm × 3 cm Fold along the dotted line Cut along the dotted line Needle ThermoCole Fig. 12. 15 3. This activity should be performed at night. Go to a room where there is a fluorescent tube light. Charge a balloon. Switch off the tube light so that the room is completely dark. Bring the charged balloon near the tubelight. You should see a faint glow. Move the balloon along the length of the tube and observe how the glow changes. Caution: Do not touch the metal parts of the tube or the wires connecting the tube with the mains. 4. Find out if there is an organisation in your area which provides relief to those suffering from natural disaster. Enquire about the type of help they render to the victims of earthquakes. Prepare a brief report on the problems of the earthquake victims. For more information on these topics, visit: - science. howstuffworks. com/lightning. htm - science. howstuffworks. com/earthquake. htm 2024-25


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LL IIGGHHTT T he world is largely known through comb and close all its openings the senses. The sense of sight except one in the middle. You can is one of the most important use a strip of black paper for senses. Through it we see mountains, this purpose. Hold the comb rivers, trees, plants, chairs, people and perpendicular to the sheet of paper. so many other things around us. We also Throw light from a torch through see clouds, rainbows and birds flying the opening of the comb from one in the sky. At night we see the moon side (Fig. 13. 1). With slight and the stars. You are able to see the adjustment of the torch and the words and sentences printed on this comb you will see a ray of light along page. How is seeing made possible? the paper on the other side of the 13. 1 What makes Things comb. Keep the comb and the torch steady. Place a strip of plane mirror Visible in the path of the light ray Have you ever thought how we see (Fig. 13. 1). What do you observe? various objects? You may say that eyes see the objects. But, can you see an object in the dark? It means that eyes alone cannot see any object. It is only when light from an object enters our eyes that we see the object. The light may have been emitted by the object, or may have been reflected by it. You learnt in Class VII that a polished or a shiny surface can act as a mirror. A mirror changes the direction of light that falls on it. Can you tell in which direction the light falling on a surface Arrangement for showing reflection will be reflected? Let us find out. After striking the mirror, the ray of 13. 2 Laws of Reflection light is reflected in another direction.


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Activity 13.1 The light ray, which strikes any surface,

[Activity] ## Activity 13. 1 The light ray, which strikes any surface, is called the incident ray. The ray that Fix a white sheet of paper on a comes back from the surface after drawing board or a table. Take a reflection is known as the reflected ray. 2024-25 Angles of Incidence A ray of light is an idealisation. In and Reflection reality, we have a narrow beam of light which is made up of several S. Angle of Angle of rays. For simplicity, we use the term No. Incidence (∠∠i) Reflection (∠∠r) ray for a narrow beam of light. 1. Draw lines showing the position of 2. the plane mirror, the incident ray and 3. the reflected ray on the paper with the help of your friends. Remove the mirror 4. and the comb. Draw a line making an 5. angle of 90º to the line representing the mirror at the point where the incident Do you see any relation between the ray strikes the mirror. This line is known angle of incidence and the angle of as the normal to the reflecting surface reflection. Are they approximately equal? at that point (Fig. 13. 2). The angle If the experiment is carried out carefully, it is seen that the angle of incidence is always equal to the angle of reflection. This is one of the laws of reflection. Let us perform another activity on reflection. Drawing the normal What would happen if I between the normal and incident ray is threw the light on the called the angle of incidence (∠∠i). The mirror along the normal. angle between the normal and the reflected ray is known as the angle of


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[Activity] ## Activity 13. 2 reflection (∠∠r) (Fig. 13. 3). Measure the angle of incidence and the angle of Perform ## Activity 13. 1 again. This reflection. Repeat the activity several time use a sheet of stiff paper or a times by changing the angle of chart paper. Let the sheet project a incidence. Enter the data in Table 13. 1. little beyond the edge of the Table (Fig. 13. 4). Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure Reflected Incident that the reflected ray extends to the ray ray projected portion of the paper. Bend that part of the projected portion on Normal which the reflected ray falls. Can you still see the reflected ray? Bring Angle of incidence and angle of the paper back to the original reflection 2024-25 with the Sun as the source of light position. Can you see the reflected instead of a torch. You, too, can use the ray again? What do you infer? Sun as the source of light. These activities can also be performed by making use of the Ray Streak Apparatus (available in the kit prepared by


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[Activity] ## Activity 13. 3 plane A source of light O is placed in front When the whole sheet of paper is of a plane mirror PQ. Two rays OA spread on the table, it represents one and OC are incident on it (Fig. 13. 5). plane. The incident ray, the normal at Can you find out the direction of the point of incidence and the reflected the reflected rays? ray are all in this plane. When you bend Draw normals to the surface of the paper you create a plane different the mirror PQ, at the points A and from the plane in which the incident C. Then draw the reflected rays at ray and the normal lie. Then you do not the points A and C. How would you see the reflected ray. What does it draw these rays? Call the reflected indicate? It indicates that the incident rays AB and CD, respectively. ray, the normal at the point of Extend them further. Do they meet? incidence and the reflected ray all lie Extend them backwards. Do they in the same plane. This is another law meet now? If they meet, mark this of reflection. point as I. For a viewer's eye at E Paheli and Boojho performed the (Fig. 13. 5), do the reflected rays above activities outside the classroom 2024-25 Parallel rays incident on an irregular surface Image formation in a plane mirror appear to come from the point I. Since the reflected rays do not actually meet at I, but only appear to do so, we say that a virtual image Rays reflected from irregular of the point O is formed at I. As you surface have learnt already in Class VII, such an image cannot be obtained When all the parallel rays reflected on a screen. from a rough or irregular surface are not parallel, the reflection is known as You may recall that in an image diffused or irregular reflection. formed by a mirror the left of the object


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## Remember that the diffused reflection appears on the right and the right is not due to the failure of the laws of appears on the left. This is known as reflection. It is caused by the lateral inversion. irregularities in the reflecting surface, like that of a cardboard. 13. 3 Regular and Diffused On the other hand, reflection from Reflection a smooth surface like that of a


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Activity 13.4 mirror is called regular reflection

[Activity] ## Activity 13. 4 mirror is called regular reflection (Fig. 13. 8). Images are formed by Imagine that parallel rays are regular reflection. incident on an irregular surface as shown in Fig. 13. 6. ## Remember that the laws of reflection are valid at each point of the surface. Use these laws to construct reflected rays at various points. Are they parallel to one another? You will find that these rays are reflected in different directions. (Fig. 13. 7) Regular reflection 2024-25 Do We See all Objects due to Reflected Light? Nearly everything you see around is seen due to reflected light. Moon, for example, receives light from the Sun and reflects it. That's how we see the moon. The objects which shine in the light of other objects are called illuminated objects. Can you name some other such objects? There are other objects, which give their own light, such as the Sun, fire, flame of a candle and an electric lamp. Their light falls on our eyes. That is how we see them. The objects which emit their own light are known as luminous objects. Paheli recalls having constructed a periscope as an Extended ## Activity in I have a question. Can the Class VI. The periscope makes use of reflected rays be further reflected two plane mirrors. Can you explain if incident on another mirror? how reflection from the two mirrors enables you to see objects which are Let us find out. not visible directly? Periscopes are 13. 4 Reflected Light Can be used in submarines, tanks and also Reflected Again by soldiers in bunkers to see things outside. Recall the last time you visited a hair dresser. She/he makes you sit in front of 13. 5 Multiple Images a mirror. After your hair cut is complete, You are aware that a plane mirror forms she/he holds a mirror behind you to show only a single image of an object. What you how the hair has been cut happens if two plane mirrors are used (Fig. 13. 9). Do you know how you could in combination? Let us see. see the hair at the back of your head? Mirror at the hair dresser shop 2024-25


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Activity 13.5 Can you now explain how you can

[Activity] ## Activity 13. 5 Can you now explain how you can see the back of your head at the hair Take two plane mirrors. Set them dresser's shop? at right angles to each other with This idea of number of images formed their edges touching (Fig. 13. 10). To by mirrors placed at an angle to one hinge them you can use adhesive another is used in a kaleidoscope to tape. Place a coin in between the make numerous beautiful patterns. You mirrors. How many images of the can also make a kaleidoscope yourself. coin do you see (Fig. 13. 10)? Kaleidoscope


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[Activity] ## Activity 13. 6 To make a kaleidoscope, get three rectangular mirror strips each about 15 cm long and 4 cm wide. Join them together to form a prism as shown in Fig. 13. 12(a). Fix this arrangement of mirrors in a circular cardboard tube or tube of a thick Images in plane mirror at right angle to each other chart paper. Make sure that the tube is slightly longer than the mirror Now hinge the mirrors using the strips. Close one end of the tube by adhesive tape at different angles, say a cardboard disc having a hole in 45°, 60°, 120°, 180° etc. Place some the centre, through which you can object (say a candle) in between see. To make the disc them. Note down the number of durable, paste a piece of transparent images of the object in each case. plastic sheet under the cardboard Finally, set the two mirrors parallel to each other. Find out how many images of a candle placed between them are formed (Fig. 13. 11). (a) mirrors (b) (c) Image in plane mirror parallel to each other Making a kaleidoscope 2024-25


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13.6 Sunlight — White or disc. At the other end, touching the Coloured mirrors, fix a circular plane glass plate [Fig.

13. 6 Sunlight - White or disc. At the other end, touching the Coloured mirrors, fix a circular plane glass plate. Place on this In Class VII, you learnt that the sunlight glass plate several small pieces of is referred to as white light. You also coloured glass (broken pieces of learnt that it consists of seven colours. coloured bangles). Close this end of Here is another activity (## Activity 13. 7) the tube by a ground glass plate. showing that sunlight consists of several Allow enough space for the colour colours. pieces to move around. 13. 7 What is inside Our Eyes? Your kaleidoscope is ready. When We see things only when light coming you peep through the hole, you will be from them enters our eyes. Eye is one able to see a variety of patterns in the of our most important sense organs. It tube. An interesting feature of a is, therefore, important to understand kaleidoscope is that you will never see its structure and working. the same pattern again. Designers of The eye has a roughly spherical wallpapers and fabrics and artists often shape. The outer coat of the eye is use kaleidoscopes to get ideas for new white. It is tough so that it can protect patterns. To make your toy attractive, the interior of the eye from accidents. you can wrap the kaleidoscope in a coloured paper. Its transparent front part is called


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[Activity] ## Activity 13. 7 Get a plane mirror of a suitable size. prism. As you learnt in Class VII, Place it in a bowl (katori) as shown this breaks up the light into its in Fig. 13. 13. Fill the bowl with colours, Splitting of light into its water. Put this arrangement near a colours is known as dispersion of window in such a way that direct light. Rainbow is a natural sunlight falls on the mirror. Adjust phenomenon showing dispersion. the position of the bowl so that the reflected light from the mirror falls on a wall. If the wall is not white, fix a sheet of white paper on it. Reflected light will be seen to have many colours. How can you explain this? The mirror and water form a Dispersion of light 2024-25 cornea (Fig. 13. 14). Behind the cornea, Class VII. The lens focuses light on the we find a dark muscular structure back of the eye, on a layer called retina called iris. In the iris, there is a small (Fig. 13. 14). The retina contains several opening called the pupil. The size of nerve cells. Sensations felt by the nerve the pupil is controlled by the iris. The cells are then transmitted to the brain iris is that part of eye which gives it its through the optic nerve. There are two distinctive colour. When we say that a kinds of cells- person has green eyes, we refer actually (i) cones, which are sensitive to bright to the colour of the iris. The iris controls light and the amount of light entering into the (ii) rods, which are sensitive to dim light. eye. Let us see how. Cones sense colour. At the junction of the optic nerve and the retina, there Ciliary are no sensory cells, so no vision is muscle possible at that spot. This is called the Iris blind spot. Its existence can be Optic Nerve demonstrated as follows: Lens


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[Activity] ## Activity 13. 9 Cornea Make a round mark and a cross on Retina a sheet of paper with the spot to the right of the cross (Fig. 13. 15). The Human eye distance between two marks may be Caution: For this activity, never use 6-8 cm. Hold the sheet of paper at a laser torch. an arm's length from the eye. Close


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Activity 13.8 your left eye. Look continuously at

[Activity] ## Activity 13. 8 your left eye. Look continuously at the cross. Move the sheet slowly Look into your friend's eye. Observe towards you, keeping your eye on the size of the pupil. Throw light on the cross. What do you find? Does her eye with a torch. Observe the the round mark disappear at some pupil now. Switch off the torch, and point? Now close your right eye. observe her pupil once again. Do Look at the round mark now and you notice any change in the size repeat the activity. Does the cross of the pupil? In which case was the disappear? The disappearance of the pupil larger? Why do you think it cross or the round mark shows that was so? there is a point on the retina which In which case do you need to cannot send messages to the brain allow more light in the eye, when when light falls on it. the light is dim or bright? Behind the pupil of the eye is a lens which is thicker in the centre. What kind of lens is thicker at the centre? Demonstration of blind spot Recall what you learnt about lenses in 2024-25 The impression of an image does not Nature has provided eyes with vanish immediately from the retina. It eyelids to prevent any object from persists there for about 1/16th of a entering the eye. Eyelids also shut out second. So, if still images of a moving light when not required. object are flashed on the eye at a rate Eye is such a wonderful instrument faster than 16 per second, then the eye that it (normal) can clearly see distant perceives this object as moving. objects as well as objects nearby. The minimum distance at which the eye can


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Activity 13.10 see objects distinctly varies with age. The

[Activity] ## Activity 13. 10 see objects distinctly varies with age. The most comfortable distance at which one Get a square piece of cardboard of can read with a normal eye is about size 6-8 cm. Make two holes as 25 cm. shown in Fig. 13. 16. Thread a Some persons can see objects close string through the two holes. Draw/ to them clearly but cannot see distant paste a cage on one side of the objects so clearly. On the other hand, cardboard and a bird on the other some persons cannot see objects nearby side. Twist the string and make the clearly but they can see distant objects card twirl rapidly. Do you see the quite well. With suitable corrective bird in the cage? lenses, these defects of the eye can be corrected. Sometimes, particularly in old age, eyesight becomes foggy. It is due to the Reverse side Front side of eye lens becoming cloudy. When it of cardboard cardboard happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe. It is possible to treat this defect. The opaque lens is removed and a new artificial lens is inserted. Modern technology has made this procedure simpler and safer. 13. 8 Care of the Eyes It is necessary that you take proper care Bird in cage of your eyes. If there is any problem you should go to an eye specialist. Have The movies that we see are actually a regular checkup- a number of separate pictures in - If advised, use suitable spectacles. proper sequence. They are made to move across the eye usually at the rate l Too little or too much light is bad of 24 pictures per second (faster for eyes. Insufficient light causes than 16 per second). So, we see a eyestrain and headaches. Too much moving picture. light, like that of the Sun, a powerful 2024-25 Did you know? Animals have eyes shaped in different ways. Eyes of a crab are quite small but they enable the crab to look all around. So, the crab can sense even if the enemy approaches from behind. Butterflies have large eyes that seem to be made up of thousands of little eyes (Fig. 13. 17). They can see not only Eyes in the front and the sides but the back as well. A night bird (owl) can see very well in the night but not during the day. On the other hand, day light birds (kite, eagle) can see well during the day but not in the night. The owl has a large cornea and a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods and only a few cones. The day birds on the other hand, have more Eyes of a butterfly cones and fewer rods. lamp or a laser torch can injure the vegetables (such as spinach) and cod retina. liver oil are rich in vitamin A. Eggs, milk, curd, cheese, butter and fruits - Do not look at the Sun or a powerful such as papaya and mango are also rich light directly. in vitamin A. - Never rub your eyes. If particles of