Question

A prism disperses a ray of white light while a rectangular glass slab does not. This is because-

Answer 1

1. A light ray is refracted when it passes from one medium to another at an angle and its speed changes. Because different wavelengths of light travel through a medium at different speeds, the amount of bending is different for different wavelengths.
2. When light passes through a glass, it encounters two surfaces- one entering and the other leaving. It slows down at the first interface and speeds back up at the second.
3. If the two interface surfaces are parallel as in the case of a rectangular glass slab, all of the bending that took place at the first interface is exactly reversed at the second, 'undoing' the effect of the first interface; so although the emerging ray of light is displaced slightly from the entering ray and all wavelengths that separated interface are recombined.
4. But in the case of a prism, the two surfaces are not parallel. So the effect of the first interface is not reversed and the colors separated at the interface continue along different paths, upon leaving the glass.