(a) The splitting up of white light into its constituent colours on passing through a refractins medium like a glass prism is called dispersion of light.
The dispersion of white light occurs because diflerent colours of light bend through different angles with respect to the incident ray, as they pass through a prism. The red light bends the least while the violet the most as shown below.
(b) A glass slab acts as a combination of two identical glass prisms.
The second prism can be considered to be placed in an inverted position with respect to the first. The first prism splits the white light into its seven colour components. When these colour components fall on the second prism, it recombines them to form white light.
