(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.
Draw a suitable diagram also. 

(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?

(a) (i) The splitting up of white light into seven colours on passing through a transparent medium like a glass prism is called dispersion of light.

(ii) The dispersion of white light occurs because the angle of refraction of lights of different colours is different when passing through the glass prims.



(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 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 refraction 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 given an impression of white light.