For which colour of light is the refractive index of a prism material the minimum and for which it is the maximum?

The refractive index of a prism material is minimum for red light and maximum for violet light.

Reason — The refractive index depends on the wavelength of light: longer wavelengths (like red) are refracted less and hence have lower refractive index, while shorter wavelengths (like violet) are refracted more and thus have higher refractive index.