Question -
Answer -
The colour develops because of the presenceof electrons in the anionic sites. These electrons absorb energy from thevisible part of radiation and get excited.
For example, when crystals of NaCl are heated inan atmosphere of sodium vapours, the sodium atoms get deposited on the surfaceof the crystal and the chloride ions from the crystal diffuse to the surface toform NaCl with the deposited Na atoms. During this process, the Na atoms on thesurface lose electrons to form Na+ ions and the releasedelectrons diffuse into the crystal to occupy the vacant anionic sites. Theseelectrons get excited by absorbing energy from the visible light and impartyellow colour to the crystals.