Question

3) What
are
f centres ?​

Answer 1

Answer:

we don't know about it....

Answer 2

$answer$

An F-center, Farbe center or color center is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more unpaired electrons. Electrons in such a vacancy tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored.