Refraction
The direction of propagation of an obliquely incident (0°< i
< 90°) ray of light that enters the other medium, changes at the
interface of the two media. This phenomenon is called refraction of light.
Snell
experimentally obtained the following laws of refraction:
(i)
The incident ray, the refracted ray and the normal to the interface at the
point of incidence, all lie in the same
plane.
(ii)
The ratio of the sine of the angle of incidence to the sine of angle of
refraction is constant. Remember that the angles of incidence (i) and
refraction (r) are the angles that the incident and its refracted ray
make with the normal, respectively. We have
= µ21
Where
µ21 is a constant, called the refractive index of the
second medium with respect to the first medium. This equation is known as
Snell’s law.
The refractive index is defined as the
ratio between the speed of light in vacuum to that in the medium.
Refractive index = c/v (where c is speed in
vacuum and v is speed in medium).
Let v1 be
the speed of light in medium 1 and v2 be the speed of light in medium 2
then,
- The refractive index of medium 2
with respect to medium 1 is given by the ratio of the speed of light in medium
1 and the speed of light in medium 2.
Where n21 is the refractive index of medium 2 with respect to medium 1. - The refractive index of medium 1
with respect to medium 2 is represented as n12.
Where n12 is the
refractive index of medium 1 with respect to medium 2.
If medium 1 is vacuum or air, then the refractive index of medium
2 is considered with respect to vacuum. This is called the absolute refractive index of the medium.
If c is the speed of light in the air and v is the speed of light in
any medium then refractive index nm of the medium would be
If µ21 > 1, r < i, i.e., the
refracted ray bends towards the normal. In such a case medium 2 is said to be optically
denser (or denser, in short) than medium 1. On the other hand, if µ21
<1, r > i, the refracted ray bends away from the normal.
This is the case when incident ray in a denser medium refracts into a rarer
medium.
If n21 is the
refractive index of medium 2 with respect to medium 1 and n12
the refractive index of medium 1 with respect to medium 2, then it should be
clear that
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