$(a)$ What is dispersion of white light? What is the cause of such dispersion? Draw a diagram to show the dispersion of white light by a glass prism.
$(b)$ $A$ glass prism is able to produce a spectrum when white light passes through it,but a glass slab does not produce any spectrum. Explain why this is so.

(N/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 a vacuum is the 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 light occurs at the first surface of the prism where its constituent colours are deviated through different angles. At the second surface,these split colours undergo further refraction and get separated more. However,in a rectangular glass slab,the refraction of light takes place at two parallel surfaces. At the first surface,although the white light splits into its constituent colours on refraction,these split colours,upon undergoing refraction at the second parallel surface,emerge as a parallel beam,which gives the impression of white light.