Define F - centres.

F centre is a type of crystallographic defect. In this defect an anionic vacancy in a crystal is filled by one or more electrons, based on charge on the missing ion in the crystal. Electrons which are present in this vacancy tend to absorb light in a visible spectrum in such a way that the transparent material seems to be coloured. Example of this crystal is NaCl. Application: Used in identifying compounds like Zinc oxide.