Ionic solids,which have anionic vacancies due to metal excess defect,develop colour. Explain with the help of a suitable example.

(N/A) The colour develops because of the presence of electrons in the anionic sites,known as $F$-centers. These electrons absorb energy from the visible part of radiation and get excited.
For example,when crystals of $NaCl$ are heated in an atmosphere of sodium vapours,the sodium atoms get deposited on the surface of the crystal and the chloride ions $(Cl^-)$ from the crystal diffuse to the surface to form $NaCl$ with the deposited $Na$ atoms. During this process,the $Na$ atoms on the surface lose electrons to form $Na^+$ ions and the released electrons diffuse into the crystal to occupy the vacant anionic sites. These electrons get excited by absorbing energy from the visible light and impart yellow colour to the crystals.