Page 164 - Science Class 08
P. 164
Refrac on of light is a phenomenon wherein No, now the pencil will be displaced to some
light bends and travels from one transparent other extent. The extent of displacement is
substance to another. different for different medium. This shows that
Let us understand this concept in depth with light does not travel in the same direc on in all
the given illustra on: medium. The direc on of propaga on of light
while travelling obliquely changes from one
medium to another. This phenomenon is
known as refrac on of light. We can define
refrac on as the phenomenon of bending of
light when it passes from one substance to
another. Rainbow, is a real-life example of
refrac on. Sunrise and sunset is a result of
atmospheric refrac on.
So, have you ever observed the bo om of a
REFRACTIVE INDEX
thick glass slab when a printed paper is kept
below it? You will find that the printed ma er The word ‘index’ means a physical feature in
seems to be raised. Similarly, when a pencil is terms of a standard in Physics. So, we can
partly immersed in water in a glass tumbler, it define the refrac ve index of a medium in
appears to be displaced from its original rela on to the other medium as under :
posi on at the surface. Did you ever think why The ra o of velocity of light in one medium to
does it happen? Why can’t it appear to be at its velocity in another medium is called the
the normal posi on? refrac ve index of the second medium.
Let us understand this with the help of the case In the other words:
of par ally immersed pencil in water:
Consider a ray of light travelling from medium 1
into medium 2 with a speed (velocity) V₁ in
medium 1 and V₂ in medium 2. The refrac ve
index of medium 1 with respect to medium 2 is
given by the ra o of the speed of light in
medium 2 and the speed of light in medium 1,
by the symbol n₁₂.
Speed of light in medium 2 (V₂)
Laws of Refrac on n₁₂ =
Speed of light in medium 1 (V₁)
The pencil seems to be displaced from its
By the same argument, the refrac ve index of
original posi on because light reaching our
medium 2 with respect to medium 1 is
eyes from the por on of the pencil inside
represented as n₂₁.
water comes from a different direc on. Due to
this reason, the pencil and printed ma er seem n₂₁ = Speed of light in medium 1 (V₁)
to be displaced from the original posi on. Speed of light in medium 2 (V₂)
What if the water in the tumbler is replaced by An important point about refrac on index is:
kerosene or turpen ne? Will the pencil v A higher refrac ve index of a medium
appears to be displaced to the same extent? indicates larger bending of the light-ray.
164 Science-8
