Hi everyone. I have the following question: on passivated copper sheets used under high electric fields I found some color patterns I can't really explain. A possible explanation would be that there are some thin layers of material on top of pure copper, for instance the copper dioxide coming from the passivation, whose thickness changes, thus inducing interference patterns. However, the equation for thin film interference (e.g. en.wikipedia.org/wiki/Thin-film_interference) depends on the angle of incidence of light: 2*n*d*cosθ = mλ, where: n = refraction index of the material d = thickness of the layer θ = incidence angle of incoming light λ = light wavelength m = integer number So my assumption is that, if it really were all about thin layers of some material, depending on my position I should see different color patterns, and at some spots none! However, as the colors on the surface of the copper sheets remain unchanged, no matter what position I'm looking from, I think it's not about thin layers, rather about pollutants. Can somebody give me a clue? Thank you very much!