A narrow beam of white light passes through a slab that has...

When a narrow beam of white light enters a glass slab with parallel faces at an angle, it undergoes refraction at the first surface. The refractive index of the material varies for different wavelengths of light, causing different colours to refract at different angles. Thus, inside the slab, the white light separates into its constituent colours.

Upon reaching the opposite parallel face, these separated colours refract again. Since the two faces of the slab are parallel, the emergent rays of all colours become parallel to the original incident ray. The angular dispersion produced at the first surface is exactly cancelled at the second surface. Because the parallel rays of different colours are very close to each other, they overlap and the emergent beam appears white.