Let me explain,
In the company I am currently working for, there are many tube cutting machines. Naturally, there are many tubes lying around the place, mainly steel, aluminium or copper (imagine e.g. a 6 meter long tube with a circular section and 3 millimeters width). Some days ago I casually picked up a short copper tube and gazed through it. I saw a series of concentric annuli of different colours. The colours were the same as copper, but with different brightness: darker towards my eye, brighter near the opposite boundary.
I was surprised by the homogeneity of the annuli colour, and by the clear boundary between the annuli. Moreover, I think they had the same width, but I did not check accurately. The background that I could see at the end of the tube was only a little relevant to this effect, so assume that the tube is pointed towards a white wall. I checked some other tube, and the same happens with aluminium and with tubes of different lengths (in this case the number of annuli depends on the tube's length).
So my question is: Why do you see such annuli at all, and why their properties are as I described?