Question

# The refractive index of glass is maximum for:(A) Red colour(B) Yellow colour(C) Violet colour(D) Green colour

Hint: For visible light $\lambda$ is smallest for violet and greatest for red light.

Complete step by step solution: According to Cauchy equation
$\mu = A + \dfrac{B}{{{\lambda ^2}}}$
$\mu \to$refractive index
$\lambda \to$wavelength
For visible light $\lambda$ is smallest for violet light and greatest for red light.

Hence $\dfrac{B}{{\lambda _{red}^2}} < \dfrac{B}{{\lambda _{violet}^2}}$
Thus, ${\mu _{red}} < {\mu _{violet}}$
i.e. µ is maximum for violet colour

Option (C) is correct.

Note: The refractive index of glass ${n_g}$ is 1.52 and that of water ${n_w}$ is 1.33. Since the refractive index of glass is higher than the water, the speed of light in water is faster than the speed of light through glass. The refractive index provides a measure of the relative speed of light in different mediums.