# Regarding colour visibility due to F-centre

Below shown image is a cut out from the NCERT book.

Referring to the highlighted sentences: I know that an $$\ce{e^-}$$ absorbs and emits light of a particular wavelength when bound in an atom and corresponding to that wavelength a particular colour is visible.

But in the case of F-centres as NCERT explains..... that an $$\ce{e^-}$$ in the F-centre when excites(i.e. absorbs a photon of a particular wavelength) and then de-excites, emits that photon and due to that ionic crystals possess some color.

But here comes the problem, how can an free $$\ce{e^-}$$ (present in the F-centre) excite? What's the phenomenon here which is going on? I am not able to understand that how will an $$\ce{e^-}$$ present in the anionic vacancies excite? Overall how F-centres impart color?

• It isn’t a free electron - it is bound to the point defect. Dec 25, 2022 at 17:29

A "sea" of free electrons looks silvery, i.e., reflective, as in solid and liquid metals, such as aluminum or gallium, and even in ionized dissolved metals, such as sodium in $$\ce{NH3}$$ (although the $$\ce{Na+ in NH3}$$ looks blue, at first, as electrons are interspersed by $$\ce{NH3}$$ molecules, as it becomes more saturated, it appears metallic, i.e., specularly reflective). The key thing to understand is that delocalized electrons can reflect EMR.