Question

(a) The refractive index of glass is 1.5. What is the speed of light in glass? (Speed of light in vacuum is $3.0 × 10^8 m s^{−1}$)
(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?

Answer 1

(a) Refractive index of glass, $μ = 1.5$
Speed of light, $c = 3 × 10^8 m/s$
Speed of light in glass is given by the relation,
$v= \frac{c }{ μ}$
$= \frac{3 × 10^8 }{ 1.5} = 2 × 10^8 m/s$
Hence, the speed of light in glass is $2 × 10^8 m/s$.

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(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 the red light in the prism.