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 $$ 300 $$ millions $$ {\text{ms}}^{-1} $$ . On entering water if slows down to $$ 225 $$ million $$ {\text{ms}}^{-1} $$ . Calculate the refractive index of water .

Answer 1

(a) Laws of refraction
First law : According to the first law of refraction , the incident ray , the refracted ray and the normal at the point of incidence all lie in the same plane.
Second law : According to the second law of refraction , the ratio of the sine of angle of incidence to the sine of refraction is constant for a given pair of media.
(b) Refractive index of substance . The ratio of speed of light in vacuum to the speed of light in a medium , is called the refractive index of that medium .
(c) Speed of light in air = $$ 300 $$ million m /sec
Speed of light in water = $$ 225 $$ million m/sec

We know that
Refractive index of water = $$ \dfrac{\text{Speed of light in air}}{\text{Speed of light in water}} $$
Refractive index of water = $$ \dfrac{300 {\text{ million m/s}}}{225{\text {million m/s}}} = 1.33 $$