Question

What is the cause of dispersion.and why dispersion is not shown rectangular glass slab

Answer 1

A light ray is refracted (bent) when it passes from one medium to another at an angle and its speed changes. At the interface, it is bent in one direction if the material it enters is denser (when light slows down) and in the OTHER direction if the material is less dense (when light speeds up). Because different wavelengths (colors) of light travel through a medium at different speeds, the amount of bending is different for different wavelengths. Violet is bent the most and red the least because violet light has a shorter wavelength, and short wavelengths travel more slowly through a medium than longer ones do. Because white light is made up of ALL visible wavelengths, its colors can be separated (dispersed) by this difference in behavior.

When light passes through glass, it encounters TWO interfaces--one entering and the other leaving. It slows down at the first interface and speeds back up at the second. If the two interface surfaces are parallel to each other, as in a 'slab' of glass, all of the bending (and dispersion) that takes place at the first interfaces 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, it travels in the same direction as the incoming ray and all wavelengths that separated at the first interface are re-combined.

If the second interface is NOT parallel to the first, as in a prism, the effects of the first interface are NOT reversed and the colors separated at that interface continue along different paths upon leaving the glass.

Cause of dispersion:When light passes through a physical medium, different wavelengths tend to interact differently with the atoms. This results in a variation of the effective refractive index of the medium as the wavelength changes. One way to view this is that the refractive index of a medium is caused by the atoms absorbing the light and then after a specific time, they re-emit the light. That delay tends to vary with the wavelength of the light. Hence the net delay of the light passing through the medium will vary with wavelength, but this will appear to an observer that the medium is slowing down light in a way that varies with its wavelength. This is what causes dispersion of the light. If the light approaches the surface of a prism at an angle to the surface, the wavefront will change direction differently depending on how much the glass slows the transit of the light. Hence if the wave contains a large number of wavelengths, the wavefront will change its direction somewhat differently versus wavelength, and so the light will appear to break up into its different colors. This is dispersion. More generally, dispersion is just the variation of speed of light with wavelength in a medium. For example, in an optical fiber, a laser pulse is spectrally broadened by the short duration of the pulse. (Just Fourier Transform) . But even this broadening of the spectrum will show up as a distortion of the pulse because the different frequencies present in the pulse will all travel at slightly different speeds, so the pulse will typically broaden further and may become quite distorted if the dispersion is too large.