Question

Why spectrum formed by prism and not by a glass slab

Answer 1

This is because a dispersion takes place in prism and light ray is refracted that is they are bent when it passes through one medium at an angle and changes its speed. The different wavelengths of light is passed at different speed this gives the color formation in prism. When the light passes through glass it has two interfaces one entering and other leaving. So the spectrum is formed by prism not by glass.

Answer 2

Explanation:

A dispersion only takes place in the prism and the light rays are refracted, i.e., the light rays get bent when they pass through one medium at an angle and change their speed. The different wavelengths of light are passed at different speeds, so this gives the colour formation in the prism. When the light passes through the glass, it has only two interfaces; one entering the glass and other leaving it. So the spectrum is formed by the prism, not by the glass.

I hope it helps.