(a) The refractive index of glass is 1.5. What is the speed of light in glass? Speed of light in vacuum is 3.0 x 10⁸ m s⁻¹) (b) Is the speed of light in glass independent of the colour of light? If not, which of the two colours red and violet travels slower in a glass prism?

Ans (a).
Refractive index of glass, μ = 1.5

Speed of light, c = 3 x 10⁸ m/s

Speed of light in glass is given by the relation,

v =c/μ

= (3×10⁸)/1.5 = 2 x 10⁸ m/s.

Hence, the speed of light in glass is 2 x 10⁸ m/s.

Ans (b).

The speed of light in glass is not independent of the colour of light.

The refractive index of a violet component of white light is greater than the refractive index of a red component. Hence, the speed of violet light is less than the speed of red light in glass. Hence, violet light travels slower than red light in a glass prism.