# Why is Copper(I) Oxide Red?

This may appear to be a chemistry problem. But, after reading the Wikipedia article on copper(I) oxide, it seems to have more to do with semiconductor-physics. For example:

… light travels almost as slowly as sound in this medium.

Is that true?

What have Kramers–Kronig relations got to do with it?

To a chemist, who was never brilliant at maths, it takes a bit of understanding. I know that copper(II) oxide (Mott–Hubbard insulator [semiconductor]) is black because of intervalence charge transfer, giving rise to the generation of a highly polarising Cu(III) species. Similarly, the non-stoichiometric form of nickel(II) oxide (Mott insulator) is black because of a Ni(III) species. Again, silver(I) oxide is black … Ag(III) species.

This model does not appear to work for copper(I) oxide because the non-stoichiometry, causing the oxidation required for the balancing of charges with the oxide ions, would give Cu(II); which, by definition, is not sufficiently polarising to produce the deep, intense colour observed. Further, the reduced Cu(I) becomes Cu(0), the pure metal.

So, why is copper(I) oxide red?

No, it is not true that light moves very slowly in Cu$_2$O. Maybe some polariton extremely close to its resonant energy (I did not look into that), but not generally.
Apparently $\rm Cu_2O$ has a band gap of about $2.1\ \rm eV$ (according to the linked wikipedia page). That means it'll absorb photons with a wavelength of less than $590.4\ \rm nm$ (just do the calculation with $E=hf$). For comparison, yellow light has a wavelength in the range $570-590\ \rm nm$. Hence we detect longer wavelengths and $\rm Cu_2O$ looks reddish.