When crystals of NaCl are heated in an atmosphere of sodium vapour, the sodium atoms are deposited on the surface of the crystal.
The Cl– ions diffuse to the surface of the crystal and
combine with Na atoms to give NaCl. This happens by loss of electron by sodium atoms to form Na+ ions. The released electrons diffuse into the crystal and occupy anionic sites. As a result the crystal now has an excess of sodium. The anionic sites occupied byunpaired electrons are called F-centre. They impart yellow colour to the crystals of NaCl.

The colour results by excitation of these electrons when they absorb energy from the visible light falling on the crystals.