Question

Why is light dispersed by a glass prism

Answer 1

Due to refraction the white light is dispersed through glass prism.

Answer 2

The refraction of light at the entry location into the rectangular glass prism would cause a little separation of the white light. ... The same can be said for red light on the upper edge of the beam. Dispersion of light provides evidence for the existence of a spectrum of wavelengths present in visible light.