Question

Why refractive index of a medium is different for different wavelengths

Answer 1

As the refractive index varies with wavelength, so will the refraction angle as light goes from one material to another. Dispersion also causes the focal length of lenses to be wavelength dependent. ... For visible light normal dispersion means that the refractive index is higher for blue light than for red.

Answer 2

The change in wavelength higher will be refraction. Hence the refractive index of the medium is directly dependent on the wavelength. Refractive index is defined as the ratio of speed of light in a vacuum to the speed of light in a medium.