Question

(a) State and explain the laws of refraction of light with the help of a labelled diagram.
(b) What is meant by the refractive index of a substance?
(c) Light travels through air at 300 million ms⁻¹. On entering water it slows down to 225 million ms⁻¹. Calculate the refractive index of water.

Answer 1

(a)
Laws of refraction of light:
(i) The incident ray, the refracted ray and the normal to the interface of two transparent mediums at the point of incidence, all lie in the same plane.
(ii) The ratio of sines of angles of incidence and refraction is a constant, for the light of a given colour and for a given pair of mediums. This law is also known as Snell’s law of refraction.



(b)
If i is the angle of incidence and r is the angle of refraction, we get:
$\frac{\sin \mathrm{i}}{\sin \mathrm{r}}$ = constant
This constant value is called the refractive index of the medium in which light enters through air. Refractive index of a medium can also be expressed in terms of speed of light as follows:

₁n₂ = $\frac{\mathrm{speed}\mathrm{of}\mathrm{light}\mathrm{in}\mathrm{medium}1}{\mathrm{speed}\mathrm{of}\mathrm{light}\mathrm{in}\mathrm{medium}2}$

Here, ₁n₂ is the refractive index of medium 2 with respect to medium 1.

(c)
Here, mediums 1 and 2 are air and water, respectively.
Now,
Refractive index of water = Speed of light in air / Speed of light in water
= 300 million ms^{−1 /} 225 million ms^{−1
= 1.33
Therefore, refractive index of water is 1.33.}