Page 165 - Science Class 08

v A lower refrac ve index of a medium indicates smaller bending of the light-ray.

Refrac ve indexes of common materials in rela on to space are given in the following table:

Medium Refrac ve Index Medium Refrac ve Index

Space 1 Kerosene 1.44
Air 1.0003 Alcohol 1.36
Water 1.33 Glass crown 1.52

Turpen ne 1.47 Diamond 2.42

DENSER AND RARE MEDIUM Here, in this figure, AO is the light ray

The refrac ve index of space (vacuum) is 1 and travelling in air and incident on glass slab at
that of air is nearly 1. So, the refrac ve indexes point O.
of other materials in rela on to vacuum are Incident N
ray
taken to be the same as in rela on to the air.
A i
The capacity of a material to refract light is P O Air Q
expressed in terms of op cal density as under: Refracted ray
v A material with a higher refrac ve index is Rectangular r Glass
glass slab N¢
said to be op cally denser. N₁
v A material with a lower refrac ve index is
R
S
said to be op cally rarer. B Air
e
v Thus, water is op cally denser than air or C
air is op cally rarer than water. N₁¢ Emergent ray
v Glass is op cally denser than water or Now, on entering the glass medium this ray
water is op cally rarer than glass. bends towards the normal NN’ that is light

REFRACTION THROUGH A GLASS SLAB ray AO gets refracted on entering the glass
medium.
Let us try to follow how light reflects in a glass
slab through the following ac vity: A er ge ng refracted, this ray now travels
through the glass slab and at point B it
comes out of the glass slab as shown in the
figure.
Fix a sheet of white paper on a drawing-board
Since ray OB goes from glass medium to air,
using drawing-pins. Place a rectangular glass
it again gets refracted and bends away from
slab in the middle of the sheet and draw its
normal N¢N₁¢ and goes in direc on BC.
outline PQRS.
Here, AO, is the incident ray and BC is the
To understand the refrac on of light through
a glass slab consider the figure given here emergent ray and they both are parallel to
each other and OB is the refracted ray.
which shows the refrac on of light through a
rectangular glass slab. Emergent ray is parallel to incident ray
because the extent of bending of the ray of

Science-8 165