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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.

Vaguely like in this picture: Yes, it's not a masterpiece, I know

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?

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  • $\begingroup$ it would be beautiful as well as illustrating if you manage to get a photo. Also, to provide us with measurements of the tubes (length and diameter)... $\endgroup$
    – Nelson Vanegas A.
    Commented Feb 26, 2021 at 20:12
  • $\begingroup$ A guess (I have never seen this): could these be interference colours, caused by different reflection angles off a thin film on the inside surface? $\endgroup$
    – user137289
    Commented Feb 26, 2021 at 23:46
  • $\begingroup$ Is there a thin layer of oil on the surface? Do you see bands of color when looking at a flat metal surface at an oblique angle? $\endgroup$
    – mmesser314
    Commented Feb 27, 2021 at 0:06
  • $\begingroup$ @NelsonVanegasA. The aluminium tube had a diameter of 50mm, a border width of 3mm and was cut to about 1.50m, otherwise I would not have been able to pick it up to my eye easily. The copper tube was about the same. I'll try to post a picture on mondey. $\endgroup$
    – kataph
    Commented Feb 27, 2021 at 11:23

2 Answers 2

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I saw this question and decide to look into a copper tube I happen to have. Here it is,

https://www.youtube.com/watch?v=YokEEcfgqg0

The answer is obviously multiple reflections. The inner most disk is made of dierct light without reflection. The first ring is made of light reflected on the inner side once. The second ring is made of light reflected twice, thus is often darker than the first ring.

The answer will be more familiar if you consider a similar quetion with two closely placed, parallel mirrors.

The answer made by Keith McClary is correct.

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The innermost annulus is a primary reflection of objects that are just out of view through the hole. If you move your eye near the edge of the tube, you will see the annulus deform into an egg shape, because the angles for primary reflection are different.
I don't see more annuli through my vacuum cleaner wand, but it is not shiny new. I guess these would be multiple reflections.

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