What are f-centers ?

Explanation

`to` f-centers are the anionic sites occupied by unpaired electrons .
`to` These impart colour to cyrstals . This colour is due to the excitation of electrons when they absorb energy from the visible light .
`to` f- centres are formed by heating alkyl halide with excess of alkali metal .
E.g : NaCl crystal heated in presence of Na-vapour yellow colour is produced due to f-centres .