(a) Dispersion of white light is splitting of light into its seven constituent colours forming a band of VIBGYOR called spectrum. Cause: White light is made up of seven colours, each colour has different speed in different media. Due to different speed, the bending ability varies and the colours split/separate.
(b) Dispersion does not take place in glass slab as two refracting surfaces are parallel. The light does not split into its constituent colours.
