Explain why :
Halogens are coloured and the colour deepens on moving down in the group from fluorine to iodine.

Explanation

Halogens are coloured due to absorption of radiations in visible region which results in excitation of outer electrons of higher energy level. The smaller size halogens absorb high energy radiation as the electrons need more energy to get excited, thus, the colour emitted is of low frequency. Going down the group higher frequency radiations are absorbed hence colour deepens going down the group. Fluorine being small absorbs violet colour and shows the comple mentary colour yellow whereas iodine absorbs yellow colour and shows comple mentary colour violet.