(a) Dispersion: The splitting of white light into its constituent seven colours is called dispersion of light.
Cause : Ordinary white light is a superposition of waves of wavelengths extending throughout the visible spectrum.
The speed of light in vacuum is same for all wavelengths, but the speed in a material substance is different for different wavelengths. As a result different colours are deviated by different angles when they pass through a prism. This is called dispersion of light.
b) In a prism, the refraction of light takes place at the two slant surfaces. The dispersion of white lightoccurs at the first surface of prism where its constituent colours are deviated through different angles. At the second surface, these split colours suffer only refraction and they get further separated. But ‘ in a rectangular glass block, the infraction of light takes place at the two parallel surfaces. At the first surfaces although the white light splits into its constituent colours on refraction, but these split colours on suffering refraction at the second surface emerge out in form of a parallel beam, which give an impression of white light.