The cause of the colours shown during the oxidation of metallic copper

During the initial stages of the oxidation of a clean metallic copper surface, and before the normal black colour of cupric oxide is finally attained, bright colours appear which pass through the first and second orders of the series of colours observed and tabulated by Newton as characteristic of thin films of air of increasing thickness. These facts have been known for a considerable time, but though the interference colours of thin films of air and those of the oxide film are produced in the same order, grave doubts have existed as to whether interference was the cause of the colours shown by the copper. This problem has been approached from measurements of the intensity of the light reflected from copper oxide films of known thickness, and the results show that interference is actually the cause of the production of the colour, though the result is complicated by the opacity of the film, the dispersion of the oxide, and the scattering of the light complementary to the film colour when the metal is burnished.