CHAPTER - 15

LIGHT

1) Light travels along a straight line :- Activity :- Light a candle and fix it on a table. Take a piece of straight pipe or a rubber tube. Look at the candle through the pipe. You can see the candle flame. If the pipe is bent, you cannot see the candle flame. This shows that light travels along a straight line.

2) Reflection of light :- When light falls on a mirror, the direction of light changes. This change in the direction of light by a mirror is called reflection of light.Activity :- Take a torch and cover its glass with a chart paper. Having a small hole. Spread a chart paper on a wooden board. Keep a plane mirror vertically on it. Direct a beam of light at an angle to the mirror. The direction of light changes. If the torch is moved slightly to either side, the direction of light also changes.

3) Image formed by a plane mirror :- i) The image is erect. ii) The image is same size as the object.iii) The image is at the same distance from the mirror as the object is in front of it.iv) The image is virtual (cannot be obtained on a screen). v) In the image the right side appears left and the left side appears

right.

4a) Spherical mirrors :- Spherical mirrors are curved mirrors.

If the reflecting is inside, the spherical mirror is a concave mirror.

If the reflecting surface is outside, the spherical mirror is a convex mirror.

Eg :- A stainless steel spoon also act like a mirror. The inner side acts like a concave mirror and the outer side acts like a convex mirror

Concave Convex

b) Image formed by Concave mirror :- i) The image formed by a concave mirror may be smaller or larger than

the object.

ii) The image may also be real (can be obtained on a screen) or virtual

(cannot be obtained on a screen).

iii) The image may inverted or erect.

Activity :- Fix a concave mirror on a screen. Light a candle and keep it at a distance of 50 cm from the mirror. Try to obtain the image of the candle flame on a screen by moving the screen. Observe the image. Then move the candle closer to the mirror at different distances and observe the images.

Distance of object from the mirror Smaller / Larger than the object Inverted / Erect Real / Virtual

50 cm

40 cm

30 cm

20 cm

10 cm

5 cm

c) Uses of Concave mirrors :- Concave mirrors are used as reflectors in torches, headlights of cars, scooters etc.

Concave mirrors are used by dentists to see enlarged images of teeth.

Concave mirrors are used by doctors for examining eyes, ears, nose and throat.

d) Image formed by Convex mirror :- i) The image formed by a convex mirror is virtual (cannot

be obtained on a screen).

ii) The image is smaller in size than the object.

e) Uses of Convex mirror :- Convex mirrors are used as side mirrors in cars, scooters etc.

Convex mirror has a wider view to help drivers to see the traffic behind them.

5a) Lenses :- Lenses are of two main types. They are Convex lenses and concave lenses.

i) Convex lens is thick in the middle and thin at the

edges. A convex lens bends light inwards. So it is called

converging lens.

ii) Concave lens is thin in the middle and thick at the edges.

A concave lens bends light outwards. So it is called

diverging lens.

b) Image formed by Convex lens :-

i) The image formed by a convex may be smaller or larger

than the object.

ii) The image may also be real (can be obtained on a screen)

or virtual (cannot be obtained on a screen).

iii) The image may inverted or erect.

c) Image formed by Concave lens :- i) The image formed by a convex mirror is virtual (cannot

be obtained on a screen).

ii) The image is smaller in size than the object.

d) Uses of lenses :- Lenses are used in spectacles, microscopes, telescopes,

cameras etc.

6a) Sunlight – White or coloured ? A rainbow is usually seen in the sky after a rain when the sun is low in the sky. A rainbow has seven colours. They are – red, orange, yellow, green, blue, indigo and violet.

Activity :-

Take a glass prism and allow a narrow beam of sunlight to fall on one side of the prism. Keep a screen on the other side of the prism. We can see that seven rainbow colours on the screen. This shows that sunlight or white light is a mixture of seven colours.

b) Mixing the rainbow colours produces white light :- If the seven rainbow colours are mixed together, it produces white light.

Activity :- Take a circular cardboard and divide it into seven segments. Paint the seven rainbow colours on it. Make a small hole in the centre of the disc. Fix the disc on the tip of a refill of a ball pen. When the disc is rotated fast, the colours get mixed and appears white in colour. This disc is known as Newton’s colour disc.