CHAPTER-16

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CHAPTER-16:

CHAPTER-16 LIGHT

WHAT MAKES THINGS VISIBLE:

WHAT MAKES THINGS VISIBLE 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. A polished or a shiny 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 will be reflected. Let us find out?

LAWS OF REFLECTION:

LAWS OF REFLECTION Fix a white sheet of paper on a drawing board or a table. Take a comb and close all its openings except one in middle. You can use strip of black paper for this purpose. Hold the comb perpendicular to the sheet of paper. Throw light from torch through the opening of the comb from one side. With slight adjustment of the torch and the comb you will see a ray of light along the paper on the other side of the comb. Keep the comb and the torch steady. Place a strip of plane mirror in the path of light ray. After striking the mirror the ray of light is reflected in another direction. The light ray, which strikes any surface, is called the incident ray. The ray that comes back from the surface after reflection is known as the reflected ray.

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A ray of light is an idealisation. In reality, we have a narrow beam of light which is made up of several rays. For simplicity, we use the term ray for a narrow beam.

A line making an angle of 90 to the line representing the mirror at the point where the incident ray strikes the mirror. This line is known as the normal to the reflecting surface at that point. The angle between the normal and incident ray is called the angle of incident. The angle between the normal and the reflected ray is known as the angle of reflection.:

A line making an angle of 90 to the line representing the mirror at the point where the incident ray strikes the mirror. This line is known as the normal to the reflecting surface at that point. The angle between the normal and incident ray is called the angle of incident. The angle between the normal and the reflected ray is known as the angle of reflection.

Do you see any relation between the angle of incidence and the angle of reflection. Are they approximately equal? 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 known as the law of reflection. Let us see another activity on reflection.:

Do you see any relation between the angle of incidence and the angle of reflection. Are they approximately equal? 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 known as the law of reflection. Let us see another activity on reflection.

ACTIVITY-2 This time use a sheet of stiff paper or a chart paper. Let the sheet project a little beyond the edge of the table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls.:

ACTIVITY-2 This time use a sheet of stiff paper or a chart paper. Let the sheet project a little beyond the edge of the table. Cut the projecting portion of the sheet in the middle. Look at the reflected ray. Make sure that the reflected ray extends to the projected portion of the paper. Bend that part of the projected portion on which the reflected ray falls.

When the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane. When you bend the paper you create a plane different from the plane in which the incident ray and the normal lie. Then you don’t see the reflected ray. It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same place. This is another law of reflection.:

W hen the whole sheet of paper is spread on the table, it represents one plane. The incident ray, the normal at the point of incidence and the reflected ray are all in this plane. When you bend the paper you create a plane different from the plane in which the incident ray and the normal lie. Then you don’t see the reflected ray. It indicates that the incident ray, the normal at the point of incidence and the reflected ray all lie in the same place. This is another law of reflection.

ACTIVITY-3 A source of light O is placed in front of a plane mirror PQ. Two rays OA and OC are incident on it. Can you find out the direction of the reflected rays? Draw normals to the surface of the mirror PQ, at the points A and C. Then draw the reflected rays at the points A and C. How would you draw these rays? Call the reflected ray AB and CD, respectively. Extend them further. Do they meet? Extend them backward. Do they meet now? If they meet, mark this point as I. For a viewer’s eye at E, do the reflected rays 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 the virtual image of the point O is formed at I.:

ACTIVITY-3 A source of light O is placed in front of a plane mirror PQ. Two rays OA and OC are incident on it. Can you find out the direction of the reflected rays? Draw normals to the surface of the mirror PQ, at the points A and C. Then draw the reflected rays at the points A and C. How would you draw these rays? Call the reflected ray AB and CD, respectively. Extend them further. Do they meet? Extend them backward. Do they meet now? If they meet, mark this point as I . For a viewer’s eye at E, do the reflected rays 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 the virtual image of the point O is formed at I.

Regular & Diffused Reflection:

Regular & Diffused Reflection Activity-4 Imagine that a parallel rays are incident on an irregular surface. 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.

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Parallel rays incident on an irregular surface Rays reflected from irregular surface

When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection. Remember that the diffused reflection is not due to the failure of the laws of reflection. It is caused by the irregularities in the reflecting surface, like that of a cardboard. On the other hand, reflection from a smooth surface like that of a mirror is called regular reflection. Images are formed by regular reflection.:

When all the parallel rays reflected from a plane surface are not parallel, the reflection is known as diffused or irregular reflection. Remember that the diffused reflection is not due to the failure of the laws of reflection. It is caused by the irregularities in the reflecting surface, like that of a cardboard. On the other hand, reflection from a smooth surface like that of a mirror is called regular reflection. Images are formed by regular reflection.

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 shines in the light of other objects are called illuminated objects. There are other objects which gives their own light, such as the sun,fire,flame of a candle & 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 object.:

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 shines in the light of other objects are called illuminated objects. There are other objects which gives their own light, such as the sun,fire,flame of a candle & 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 object.

Reflected rays can be reflected again:

Reflected rays can be reflected again Imagine you visit a hair dresser. She/he makes you sit in front of a mirror. After your hair cut is complete, she/he holds a mirror behind you to show you how the hair has been cut. Do you know how you could see the hair at the back of your head? Having constructed a periscope as an extended activity in class VI. The periscope makes use of 2 plane mirrors. Can you explain how reflection from the 2 mirrors enables you to see objects which are not visible directly? Periscopes are used in submarines, tanks, and also by soldiers in bunkers to see things outside.

Multiple Images :

Multiple Images You are aware that a plane mirror forms only a single image of an object. What happens if 2 plane mirrors are used in combination? Let us see.

ACTIVITY-5 Take 2 plane mirrors. Set them at right angles to each other with their edges touching. To hinge them you can use adhesive tape. Place a coin in between the mirrors. How many images of the coin do you see? Now hinge the mirrors using adhesive tape at different angles, say 45 degree, 60 degree, 120 degree, 180 degree, etc. Place some object (say a candle) in between them. Note down the number of images of the object in each case. Finally, set the two mirrors parallel to each other. Find out how many images of a candle placed between them are formed.:

ACTIVITY-5 Take 2 plane mirrors. Set them at right angles to each other with their edges touching. To hinge them you can use adhesive tape. Place a coin in between the mirrors. How many images of the coin do you see? Now hinge the mirrors using adhesive tape at different angles, say 45 degree, 60 degree, 120 degree, 180 degree, etc. Place some object (say a cand l e) in between them. Note down the number of images of the object in each case. Finally, set the two mirrors parallel to each other. Find out how many images of a candle placed between them are formed.

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Images in plane mirror at right angle to each other Image in plane mirror parallel to each other

Can you now explain how can you see the back of your head at the hair dresser’s shop? This idea of number of images formed by mirrors placed at an angle to one another is used in kaleidoscope to make numerous beautiful patterns. You can also make a kaleidoscope yourself.:

Can you now explain how can you see the back of your head at the hair dresser’s shop? This idea of number of images formed by mirrors placed at an angle to one another is used in kaleidoscope to make numerous beautiful patterns. You can also make a kaleidoscope yourself.

A Kaleidoscope:

A Kaleidoscope

Sunlight-White or coloured:

Sunlight-White or coloured In class VII, you learnt that the sunlight is referred to as white light. You also learnt that it consists of seven colours. Splitting of light into it’s colours is known as dispersion of light. Rainbow is a natural phenomenon showing dispersion.

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DISPERSION OF LIGHT

What is inside our eyes?:

We see things only when light coming from them enters our eyes. Eye is one of our most important organs. It is, therefore, important to understand its structure and working. The eye has a roughly spherical shape. The outer coat of the eye is white. It is tough so that it can protect the interior of the eye from accidents. Its transparent front part is called cornea. Behind the cornea, we find a dark muscular structure called iris. I n the iris, there is a small opening called the pupil. The size of the pupil is controlled by the iris. The iris is that part of eye which gives it its distinctive colour. When we say that a person has green eyes, we actually refer to the colour of iris. The iris controls the amount of light entering into the eye. Let us see how. What is inside our eyes?

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Human’s eye

ACTIVITY-6 Look into your friend’s eyes. Observe the size of the pupil. Throw light on her/his eye with a torch Observe the pupil now. Switch off the torch, and observe her pupil once again. Do you notice any change in the size of the pupil?:

ACTIVITY-6 Look into your friend’s eyes. Observe the size of the pupil. Throw light on her/his eye with a torch Observe the pupil now. Switch off the torch, and observe her pupil once again. Do you notice any change in the size of the pupil?

Behind the pupil of the eye is a lens which is thicker in the centre. What kind of lens is thicker at the centre? Recall what you learnt about lenses in class VII. The lens focuses light on the back of the eye, on a layer called retina . The retina contains several nerve cells. Sensation felt by the nerve cells are then transmitted to the brain through the optic nerve. There are two kinds of cells-: (i) cones, which are sensitive to bright light & (ii) rods, which are sensitive to dim light. Cones sense colour. At the junction of the optic nerve & the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot. Its existence can be demonstrated as follows::

Behind the pupil of the eye is a lens which is thicker in the centre. What kind of lens is thicker at the centre? Recall what you learnt about lenses in class VII. The lens focuses light on the back of the eye, on a layer called retina . The retina contains several nerve cells. Sensation felt by the nerve cells are then transmitted to the brain through the optic nerve. There are two kinds of cells-: (i) cones, which are sensitive to bright light & (ii) rods, which are sensitive to dim light. Cones sense colour. At the junction of the optic nerve & the retina, there are no sensory cells, so no vision is possible at that spot. This is called the blind spot. Its existence can be demonstrated as follows:

ACTIVITY-7 Make a round mark and a cross on a sheet of paper with the spot to the right of the cross. The distance between 2 marks may be 6-8cm. Hold the sheet of paper at an arm’s length from the eye. Close your left eye. Look continuously at the cross. Move the sheet slowly towards you , keeping your eye on the cross. What do you find? Does the round mark disappear at some point? Now close your right eye. Look at the round mark now & repeat this activity. Does the cross disappear? The disappearance of the cross or the round mark shows that there is a point on the retina which cannot send messages to the brain when light falls on it. :

ACTIVITY-7 Make a round mark and a cross on a sheet of paper with the spot to the right of the cross. The distance between 2 marks may be 6-8cm. Hold the sheet of paper at an arm’s length from the eye. Close your left eye. Look continuously at the cross. Move the sheet slowly towards you , keeping your eye on the cross. What do you find? Does the round mark disappear at some point? Now close your right eye. Look at the round mark now & repeat this activity. Does the cross disappear? The disappearance of the cross or the round mark shows that there is a point on the retina which cannot send messages to the brain when light falls on it.

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Demonstration of blind spot

The impression of an image does not vanish immediately from the retina. It persists there for about 1/16th of a second. So, if still images of a moving object are flashed on the eye at a rate faster then 16 per second, then the eye perceives this object as moving. The movies that we see are actually a number of separate pictures in proper sequence. They are made to move across the eye usually at the rate of 24 pictures per second(faster than 16 per second). So, we see a moving picture. Nature has provided eyes with eyelids to prevent any object from entering the eyes.:

The impression of an image does not vanish immediately from the retina. It persists there for about 1/16 th of a second. So, if still images of a moving object are flashed on the eye at a rate faster then 16 per second, then the eye perceives this object as moving. The movies that we see are actually a number of separate pictures in proper sequence. They are made to move across the eye usually at the rate of 24 pictures per second(faster than 16 per second). So, we see a moving picture. Nature has provided eyes with eyelids to prevent any object from entering the eyes.

CARE OF OUR EYES:

It is necessary that you take proper care of your eyes. If there is any problem you should go to an eye specialist. Have a regular checkup-: If advised, use suitable spectacles. Too little or too much light is bad for eyes. Insufficient light causes eyestrain and headaches. Too much light, like that of sun, a powerful lamp or a laser torch can injure the retina. Do not look at the sun or a powerful light directly. Never rub your eyes. If particles of dust goes into your eyes, wash your eyes with clean water. If there is no improvement go to a doctor. Wash your eyes frequently with clean water. CARE OF OUR EYES

6) Always read at the normal distance for vision. Do not read by bringing the book too close to your eyes or keeping it too far.:

6) Always read at the normal distance for vision. Do not read by bringing the book too close to your eyes or keeping it too far.

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 thousand of little eyes. They can see not only in the front & the sides but back as well. A night bird (owl) can see very well in the night but not during the day. The owl has a large cornea & a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods & only a few cones. The day birds on the other hand, have more cones & fewer rods. :

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 thousand of little eyes. They can see not only in the front & the sides but back as well. A night bird (owl) can see very well in the night but not during the day. The owl has a large cornea & a large pupil to allow more light in its eye. Also, it has on its retina a large number of rods & only a few cones. The day birds on the other hand, have more cones & fewer rods.

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EYES OF BUTTERFLY EYES

Visually challenged persons can read & write :

Some persons, including children, can be visually handicapped. They have very limited vision to see things. Some persons cannot see at all since birth. Some persons may lose their eyesight because of disease. Such persons try to identify things by touching and listening to voices more carefully. They develop their other senses more sharply. However, additional resources can enable them to develop their capabilities further. Visually challenged persons can read & write

WHAT IS THE BRAILLE SYSTEM?:

The most popular resource for visually challenged persons is Braille. WHAT IS THE BRAILLE SYSTEM? Louis braille, himself a visually challenged person, developed a system for visually challenged persons & published it in 1821. LOUIS BRAILLE

Braille system has 63 dot patterns or characters. Each character represent a letter, a combination of letter, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. Patterns of dots to represent some English letters & some common words are shown below.:

Braille system has 63 dot patterns or characters. Each character represent a letter, a combination of letter, a common word or a grammatical sign. Dots are arranged in cells of two vertical rows of three dots each. Patterns of dots to represent some English letters & some common words are shown below.

Visually challenged people learn the braille system by beginning with letters, then special characters & letter combinations. Methods depends upon recognition by touching. Each character has to be memorised. Braille texts can be produced by hand or by machine. Typewriter-like devices & printing machines have now been developed.:

Visually challenged people learn the braille system by beginning with letters, then special characters & letter combinations. Methods depends upon recognition by touching. Each character has to be memorised. Braille texts can be produced by hand or by machine. Typewriter-like devices & printing machines have now been developed.

Some visually challenged Indians have great achievements to their credit. Diwakar, a child prodigy has given amazing performances as singer. Ravindra Jain, born completely visually challenged, obtained his sangeet prabhakar degree from Allahabad. He has shown his excellence as lyricist, singer & music composer. Lal Advani, himself visually challenged, established an association for special education & rehabilitation of disabled in India. Besides this, he represented India on Braille problems in UNESCO.:

Some visually challenged Indians have great achievements to their credit. Diwakar, a child prodigy has given amazing performances as singer. Ravindra Jain, born completely visually challenged, obtained his sangeet prabhakar degree from Allahabad. He has shown his excellence as lyricist, singer & music composer. Lal Advani, himself visually challenged, established an association for special education & rehabilitation of disabled in India. Besides this, he represented India on Braille problems in UNESCO.

Helen A. Keller, an American author & lecturer, is perhaps the most well known & inspiring visually challenged persons. She lost her sight when she was only 18 months old. But because of her resolve & courage she could complete her graduation from a university. She wrote a number of books including The story of my life(1903).:

Helen A. Keller, an American author & lecturer, is perhaps the most well known & inspiring visually challenged persons. She lost her sight when she was only 18 months old. But because of her resolve & courage she could complete her graduation from a university. She wrote a number of books including T he story of my life(1903).

THE END MADE BY-AMAN KUMAR:

THE END MADE BY-AMAN KUMAR

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