To explain this, let us take an example of NaCl. When NaCl crystal is heated in presence of Na vapour, some Cl^- ions leave their lattice sites to combine with Na to form Na⁺ (Na⁺ + Cl^-⇒ NaCl) then diffuse into the crystal to occupy the anion vacancies. These sites are called as the F- centres. These electrons absorb energy from the visible light, get excited to higher energy level and when they fall back to the ground state they impart yellow colour to the NaCl crystal.