$(a)$ The refractive index of glass is $1.5$. What is the speed of light in glass? (Speed of light in vacuum is $3.0 \times 10^{8} \; m/s$).
$(b)$ Is the speed of light in glass independent of the colour of light? If not,which of the two colours,red or violet,travels slower in a glass prism?

(N/A) Given: Refractive index of glass,$\mu = 1.5$. Speed of light in vacuum,$c = 3.0 \times 10^{8} \; m/s$.
The speed of light in a medium $(v)$ is given by the relation: $v = \frac{c}{\mu}$.
Substituting the values: $v = \frac{3.0 \times 10^{8}}{1.5} = 2.0 \times 10^{8} \; m/s$.
Thus,the speed of light in glass is $2.0 \times 10^{8} \; m/s$.
$(b)$ No,the speed of light in glass is not independent of the colour of light. The refractive index of glass depends on the wavelength (colour) of light. The refractive index for violet light is greater than that for red light. Since $v = \frac{c}{\mu}$,a higher refractive index results in a lower speed. Therefore,violet light travels slower than red light in a glass prism.