# (a) The refractive index of glass is 1.5. What is the speed of light in glass?

Question:

(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 × 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?

Solution:

(a) Refractive index of glass, μ = 1.5

Speed of light, c = 3 × 108 m/s

Speed of light in glass is given by the relation,

$v=\frac{c}{\mu}$

$=\frac{3 \times 10^{8}}{1.5}=2 \times 10^{8} \mathrm{~m} / \mathrm{s}$

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

(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.