hesc113.pdf

LIGHT 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 s...

LIGHT 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 page. How is seeing made possible? comb you will see a ray of light along 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 Fig. 13.1 : 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. 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  A ray of light is an idealisation. In Table 13.1 : Angles of Incidence reality, we have a narrow beam of and Reflection light which is made up of several S. Angle of Angle of rays. For simplicity, we use the term No. Incidence (∠∠i) ∠r) Reflection (∠ 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. Fig. 13.2 : 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 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 Normal that part of the projected portion on which the reflected ray falls. Can you still see the reflected ray? Bring Fig. 13.3 : Angle of incidence and angle of the paper back to the original reflection 166 SCIENCE 2024-25  position. Can you see the reflected with the Sun as the source of light ray again? What do you infer? instead of a torch. You, too, can use the 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 NCERT). Boojho remembered that in Class VII, he had studied some features of the image of an object formed by a plane mirror. Paheli asked him to recall (a) those features: (i) Was the image erect or upside down? (ii) Was it of the same size as the object? (iii) Did the image appear at the same distance behind the mirror as the object was in front of it? (iv) Could it be obtained on a screen? Let us understand a little more about (b) the formation of an image by a plane Fig. 13.4 (a), (b) : Incident ray, reflected ray and the normal at the point mirror in the following way: of incidence lie in the same plane Activity 13.3 When the whole sheet of paper is A source of light O is placed in front spread on the table, it represents one of a plane mirror PQ. Two rays OA plane. The incident ray, the normal at and OC are incident on it (Fig. 13.5). the point of incidence and the reflected Can you find out the direction of ray are all in this plane. When you bend the reflected rays? the paper you create a plane different Draw normals to the surface of from the plane in which the incident the mirror PQ, at the points A and ray and the normal lie. Then you do not C. Then draw the reflected rays at see the reflected ray. What does it the points A and C. How would you indicate? It indicates that the incident draw these rays? Call the reflected ray, the normal at the point of rays AB and CD, respectively. incidence and the reflected ray all lie Extend them further. Do they meet? in the same plane. This is another law Extend them backwards. Do they of reflection. meet now? If they meet, mark this Paheli and Boojho performed the point as I. For a viewer’s eye at E above activities outside the classroom (Fig. 13.5), do the reflected rays LIGHT 167 2024-25  Fig. 13.6 : Parallel rays incident on an irregular surface Fig. 13.5 : 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 Fig. 13.7 : 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 You may recall that in an image not parallel, the reflection is known as formed by a mirror the left of the object diffused or irregular reflection. appears on the right and the right Remember that the diffused reflection appears on the left. This is known as is not due to the failure of the laws of lateral inversion. reflection. It is caused by the irregularities in the reflecting surface, 13.3 Regular and Diffused like that of a cardboard. On the other hand, reflection from Reflection a s m o o t h s u r face like that of a 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) Fig. 13.8 : Regular reflection 168 SCIENCE 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 you how the hair has been cut only a single image of an object. What (Fig. 13.9). Do you know how you could happens if two plane mirrors are used see the hair at the back of your head? in combination? Let us see. Fig. 13.9 : Mirror at the hair dresser shop LIGHT 169 2024-25 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 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 Fig. 13.10 : Images in plane mirror at right cardboard tube or tube of a thick angle to each other chart paper. Make sure that the tube Now hinge the mirrors using the is slightly longer than the mirror adhesive tape at different angles, say strips. Close one end of the tube by 45°, 60°, 120°, 180° etc. Place some a cardboard disc having a hole in object (say a candle) in between the centre, through which you can them. Note down the number of see [Fig. 13.12(b)]. To make the disc images of the object in each case. durable, paste a piece of transparent Finally, set the two mirrors plastic sheet under the cardboard parallel to each other. Find out how many images of a candle placed between them are formed (Fig. 13.11). (a) mirrors (b) (c) Fig. 13.11 : Image in plane mirror parallel to each other Fig. 13.12 : Making a kaleidoscope 170 SCIENCE 2024-25  disc. At the other end, touching the 13.6 Sunlight — White or mirrors, fix a circular plane glass Coloured plate [Fig. 13.12(c)]. 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 you peep through the hole, you will be We see things only when light coming able to see a variety of patterns in the from them enters our eyes. Eye is one tube. An interesting feature of a of our most important sense organs. It kaleidoscope is that you will never see is, therefore, important to understand the same pattern again. Designers of its structure and working. wallpapers and fabrics and artists often The eye has a roughly spherical use kaleidoscopes to get ideas for new shape. The outer coat of the eye is patterns. To make your toy attractive, white. It is tough so that it can protect you can wrap the kaleidoscope in a the interior of the eye from accidents. coloured paper. Its transparent front part is called 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 Fig. 13.13 : Dispersion of light LIGHT 171 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 Cornea Activity 13.9 Make a round mark and a cross on Retina a sheet of paper with the spot to the Fig. 13.14 : Human eye right of the cross (Fig. 13.15). The 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 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? Fig. 13.15 : Demonstration of blind spot Recall what you learnt about lenses in 172 SCIENCE 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 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 Fig. 13.16 : 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 l 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 LIGHT 173 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 in the front and the sides but the Eyes 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 Fig. 13.17 : 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, l Do not look at the Sun or a powerful milk, curd, cheese, butter and fruits such as papaya and mango are also rich light directly. in vitamin A. l Never rub your eyes. If particles of dust go into your eyes, wash your 13.9 Visually Impaired eyes with clean water. If there is no Persons Can Read and improvement go to a doctor. Write l Always read at the normal distance for vision. Do not read by bringing Some persons, including children, can the book too close to your eyes or be visually impaired. They have very keeping it too far. limited vision to see things. Some You learnt about balanced diet in persons cannot see at all since birth. Class VI. If food is deficient in some Some persons may lose their eyesight components, eyes may also suffer. Lack because of a disease or an injury. Such of vitamin A in foodstuff is responsible persons try to identify things by for many eye troubles. Most common touching and listening to voices more amongst them is night blindness. carefully. They develop their other One should, therefore, include in the senses more sharply. However, diet components which have vitamin A. additional resources can enable them Raw carrots, broccoli and green to develop their capabilities further. 174 SCIENCE 2024-25  Non-optical and optical aids for visually impaired Non-optical aids include visual aids, tactual aids (using the sense of touch), auditory aids (using the sense of hearing) and electronic aids. Visual aids, can magnify words, can provide suitable intensity of light and material at proper distances. Tactual aids, including Braille writer slate and stylus, help the visually challenged persons in taking notes, reading and writing. Auditory aids include cassettes, tape recorders, talking books and other such devices. Electronic aids, such as talking calculators and computers, are also available for performing many computational tasks. Closed circuit television, also an electronic aid, enlarges printed material with suitable contrast and illumination. Nowadays, use of audio CDs and voice boxes with computers are also very helpful for listening to and writing the desired text. Optical aids include bifocal lenses, contact lenses, tinted lenses, magnifiers and telescopic aids. While the lens combinations are used to rectify visual limitations, telescopic aids are available to view chalkboard and class demonstrations. 13.10 What is the Braille Braille system has 63 dot patterns or System? characters. Each character represents a letter, a combination of The most popular resource for visually letters, a common word or a challenged persons is Braille. grammatical sign. Dots are arranged in cells of two vertical rows of three Louis Braille, himself a visually dots each. challenged person, developed a Patterns of dots to represent some system for visually challenged persons English letters and some common and published it in 1821. words are shown below. Louis Braille Fig. 13.18 : Example of dot patterns used in Braille System The present system was adopted in These patterns when embossed on 1932. There is Braille code for common Braille sheets help visually languages, mathematics and scientific challenged persons to recognise words notation. Many Indian languages can by touching. To make them easier to be read using the Braille system. touch, the dots are raised slightly. LIGHT 175 2024-25  Visually impaired people learn the character has to be memorised. Braille Braille system by beginning with texts can be produced by hand or by letters, then special characters and machine. Typewriter-like devices and letter combinations. Methods depend printing machines have now been upon recognition by touching. Each developed. Some visually impaired Indians have great achievements to their credit. Diwakar, a child prodigy has given amazing performances as a singer. Ravindra Jain, born completely visually impaired, obtained his Sangeet Prabhakar degree from Allahabad. He had shown his excellence as a lyricist, singer and music composer. Lal Advani, himself visually impaired, established an Helen A. Keller Association for special education and rehabilitation of disabled in India. Besides this, he represented India on Braille problems in UNESCO. Helen A. Keller, an American author and lecturer, is perhaps the most well- known and inspiring visually challenged person. She lost her sight when she was only 18 months old. But because of her resolve and courage she could complete her graduation from a university. She wrote a number of books including The Story of my Life (1903). 176 SCIENCE 2024-25 KEYWORDS WHAT YOU HAVE LEARNT ANGLE OF Ü Light is reflected from all surfaces. INCIDENCE Ü Regular reflection takes place when light is ANGLE OF incident on smooth, polished and regular REFLECTION surfaces. BLIND SPOT Ü Diffused or irregular reflection takes place from rough surfaces. BRAILLE Ü Two laws of reflection are CONES (i) The angle of incidence is equal to the angle CORNEA of reflection. (ii) Incident ray, reflected ray and the normal DIFFUSED OR drawn at the point of incidence to the IRREGULAR reflecting surface, lie in the same plane. REFLECTION Ü Image formed in a plane mirror undergoes INCIDENT RAYS lateral inversion. IRIS Ü Two mirrors inclined to each other give multiple KALEIDOSCOPE images. Ü Beautiful patterns are formed in a LATERAL INVERSION kaleidoscope because of multiple reflections. LAWS OF Ü Sunlight, called white light, consists of seven REFLECTION colours. PUPIL Ü Splitting of light into its constituent colours is REFLECTED RAYS known as dispersion. Ü Parts of the eye are cornea, iris, pupil, lens, REFLECTION retina and optic nerve. REGULAR Ü A normal eye can see nearby and distant REFLECTION objects clearly. RETINA Ü Visually impaired persons can read and write RODS using the Braille system. Ü Visually impaired persons develop their other senses more sharply to improve their interac- tion with their environment. LIGHT 177 2024-25  Exercises 1. Suppose you are in a dark room. Can you see objects in the room? Can you see objects outside the room. Explain. 2. Differentiate between regular and diffused reflection. Does diffused reflection mean the failure of the laws of reflection? 3. Mention against each of the following whether regular or diffused reflection will take place when a beam of light strikes. Justify your answer in each case. (a) Polished wooden table (b) Chalk powder (c) Cardboard surface (d) Marble floor with water spread over it (e) Mirror (f) Piece of paper 4. State the laws of reflection. 5. Describe an activity to show that the incident ray, the reflected ray and the normal at the point of incidence lie in the same plane. 6. Fill in the blanks in the following. (a) A person 1 m in front of a plane mirror seems to be _______________ m away from his image. (b) If you touch your ____________ ear with right hand in front of a plane mirror it will be seen in the mirror that your right ear is touched with ____________. (c) The size of the pupil becomes ____________ when you see in dim light. (d) Night birds have ____________ cones than rods in their eyes. Choose the correct option in Questions 7 – 8 7. Angle of incidence is equal to the angle of reflection. (a) Always (b) Sometimes (c) Under special conditions (d) Never 8. Image formed by a plane mirror is (a) virtual, behind the mirror and enlarged. (b) virtual, behind the mirror and of the same size as the object. (c) real at the surface of the mirror and enlarged. (d) real, behind the mirror and of the same size as the object. 9. Describe the construction of a kaleidoscope. 10. Draw a labelled sketch of the human eye. 178 SCIENCE 2024-25  11. Gurmit wanted to perform Activity 13.8 using a laser torch. Her teacher EXERCISES advised her not to do so. Can you explain the basis of the teacher’s advise? 12. Explain how you can take care of your eyes. 13. What is the angle of incidence of a ray if the reflected ray is at an angle of 90° to the incident ray? 14. How many images of a candle will be formed if it is placed between two parallel plane mirrors separated by 40 cm? 15. Two mirrors meet at right angles. A ray of light is incident on one at an angle of 30° as shown in Fig. 13.19. Draw the reflected ray from the second mirror. Fig. 13.19 16. Boojho stands at A just on the side of a plane mirror as shown in Fig. 13.20. Can he see himself in the mirror? Also can he see the image of objects situated at P, Q and R? Fig. 13.20 17. (a) Find out the position of the image of an object situated at A in the plane mirror (Fig. 13.21). (b) Can Paheli at B see this image? (c) Can Boojho at C see this image? (d) When Paheli moves from B to C, where does the image of A move? Fig. 13.21 LIGHT 179 2024-25  Extended Learning — Activities and Project 1. Make your own mirror. Take a glass strip or glass slab. Clean it and put it on a white sheet of paper. See yourself in the glass. Next put the glass slab on a black sheet of paper. Again look into the glass. In which case do you see yourself better and why? 2. Make friends with some visually impaired students. Enquire from them how they read and write. Also find out how they are able to recognise objects, hurdles and currency notes. 3. Meet an eye specialist. Get your eye sight checked and discuss how to take care of your eyes. 4. Survey your neighbourhood. Find out how many children below the age of 12 years use spectacles. Find out from their parents what, in their view, could be the reason for the weak eyesight of their children. Did You Know? Eyes can be donated by any person as an invaluable gift to visually impaired persons suffering from corneal blindness, The person may be (a) a male or female. (b) of any age. (c) of any social status. (d) using spectacles. (e) suffering from any normal disease but not AIDS, Hepatitis B or C, rabies, leukemia, lymphoma, tetanus, cholera, encephalitis. The eyes have to be donated within 4-6 hours after death at any place, home or hospital. A person who wants to donate the eyes may pledge eyes during his/her lifetime to any registered eye bank. He/she should also inform his/her relatives about this pledge so that they can take necessary action after his/her death. You can also donate a Braille kit. 180 SCIENCE 2024-25 Notes 2024-25 Notes 2024-25