Question

Why does white light splits on first face of a prism

Answer 1

White light will begin to disperse inside any medium, when the incident angle is not zero. A flat piece of window glass will do the same thing to light inside as a prism. The difference is that the dispersion is cancelled out normally in a glass with parallel faces, since light would disperse in the opposite way refracting at the same angle and look white again. On the other hand, a prism has sides that are at an angle to each other, and if the right position is chosen, white light can further disperse a second time making colors visible separately. Plane glass has the similar effect of two prisms (one upside-down compared to the other) recollimating white light.