Page 163 - Science Class 08

Take a glass tumbler and fill it more than half with water. Take a pencil and place it in the
tumbler such that a part of it is immersed in water. Observe the pencil from a side. You will see
that the straight pencil appears bent at the surface of the water. Not only this, the part of the
pencil below the surface of the water looks to be thicker when we see it from a side.
We can check this fact in the following cases as well:

(a) Rasagullaas kept in sugar syrup in a jar appear larger than their actual size when viewed
from the side of the jar.
(b) A lemon placed in a tumbler of water appears to be larger.
(c) Place a coin in a bowl of water. Observe it from a side. It will look bigger as well as raised,
i.e., it will look as if it were above the bo om of the bowl.
(d) Observe the body of a diver when he is totally under water. It will appear inflated.
(e) Take a torch and cover its glass with a thick black paper with a pin-hole in its centre. When
the torch is switched on, a thin ray of light will come out of the hole.
(f) Take some milky water in a glass and place it in a dark room. Light agarba s about the
glass and cast the torch-light on the milky water. You will see its deflec on on entering the
milky water.
What are all these phenomena due to? They are all due to refrac on.
So, we can define reflec on as under:

Deflec on of light-rays at a certain angle when they enter from one medium into another
obliquely, is called refrac on.

LAWS OF REFRACTION (leaves) the medium of refrac on again
Before reading about the laws of refrac on, we deflects and is called the emergent ray.
must know about some terms related to the We know that light travels with a speed of
phenomenon of refrac on. about 3,00,000 km per second. In vacuum, this

They are as under: speed is the highest and it is lower in air, water,
glass, kerosene and glycerine. Clearly, light
v The point where a ray of light strikes a
travels at different speeds in these mediums.
medium is the point of incidence and the
One more thing is clear that when rays of light
ray is called the incident ray.
enter from one medium into another, they
v A perpendicular drawn at the point of
deflect and their speed changes, i.e., it
incidence is called the normal.
becomes higher or lower.
v The incident ray, when it has deflected
Their deflec on (refrac on) takes place accor-
while entering the new medium, is called ding to the following two laws:
refracted ray.
v A ray of light entering a new medium
v The angle between the incident ray and the deflects towards the normal, if its speed
normal is the angle of incidence. slows down on entering the new medium.
v The angle between the refracted ray and v A ray of light entering a new medium bends
the normal is the angle of refrac on. away from the normal, — if its speed gets

v The refracted ray, when it emerges from faster on entering the new medium.

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