What is light and how to measure it

1. CHAPTER - 15
LIGHT

2. 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.

3. 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.

4. 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.

5. 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

6. 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

7. 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.

8. 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.

9. 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.

10. 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.

11. 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.

12. 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.