LIGHT NOTES PART -4
REFRACTION
When light travels from one medium to another, the speed changes, as does the wavelength.
Although the speed(v) changes and wavelength(λ) changes, the frequency(ʋ) of the light will
be constant. The direction changes only in the case of oblique incidence. The frequency,
wavelength, and speed are related by:
v=ʋλ
Refraction : The change in direction of light when it passes obliquely from one medium to
another medium is called refraction of light.
Characteristics of Refraction:
• Refraction occurs due to change in speed of light when it travels from one medium to
another of different optical density.
• Any medium through which light can travel is called an optical medium
• Types of optical medium :
i) A medium in which the speed of light is less is known as an optically denser
medium
ii) A medium in which the speed of light is more is known as an optically rarer
medium
• If light is travelling from an optically denser medium into an optically rarer medium, it
bends away from the normal. ( Fig 1.a)
• If light is travelling from an optically rarer medium into an optically denser
medium, it bends towards the normal. (Fig 1.b)
Fig 1.a Fig 1.b
Examples of refraction
( activities 10.7,10.8 and 10.9 based on examples of refraction to be learnt from
NCERT reader pg 172)
REFRACTION THROUGH A RECTANGULAR GLASS SLAB
To understand the refraction of light through a glass slab consider the figure given which
shows the refraction of light through a rectangular glass slab.
▪ EO is the light ray traveling in air and incident on glass slab at point O.
▪ The Light ray on entering the glass medium , bends towards the normal NN’ that
is light ray EO gets refracted on entering the glass medium.
▪ After getting refracted this ray now travels through the glass slab and at point
O’ it comes out of the glass slab as shown in the figure.
▪ Since ray O’H goes from glass medium to air it again gets refracted and bends
away from normal MM’ and goes in direction O’H.
▪ Here EO is the incident ray and O’H is the emergent ray and they both are
parallel to each other and OO’ is the refracted ray.
▪ Emergent ray is parallel to incident ray because the extent of bending of the ray
of light at the opposite parallel faces which are AB (air-glass interface)
and CD (glass-air interface) of the rectangular glass slab is equal and opposite.
▪ In the figure i1 is the angle of incidence, r1 is the angle of refraction and r2 is the
angle of emergence.
INFERENCE : Angle of incidence and angle of emergence are equal as emergent ray
and incident ray are parallel to each other, however the light ray has shifted
sideward (laterally) slightly.
Note:When a light ray is incident normally to the interface of two media then there
is no bending of light ray and it goes straight through the medium.
