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I got this metal phone ring stand from online. When I was meddling with it, I notice the ring started to produce iridescence. (See photo-only the centre portion, the moveable ring is not affected) I believe the mild heat from my hand caused this to happen. I left it alone on a table overnight, and the next morning, the iridescence disappeared. Te ring is back to normal.

Whenever I applied mild heat again (I placed it at the back of my vacuum cleaner for 5seconds), the iridescent will appear, but will then disappear again after I leave it overnight.

The iridescent consists of greenish, pinkish and purplish hues. It doesn’t seem to be forming at the surface, but rather it seems to be forming in the interior.

The seller claims that the ring is made up of zinc alloy, but it is magnetic. Some guesses of the material that might be in the product: zinc alloy, nickel, steel, chrome plating (the centre portion is very reflective like a mirror) Please correct me if my guesses are wrong.

I am just very perturbed by this phenomenon occurring in such simple object and am curious to find out what causes this!!

Thank you!! enter image description here

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2 Answers 2

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This is an interference phenomenon, more specifically interference of light between two thin layers with an air gap. Recall the beautiful soap bubble as an example. Whatever coating your have on this object gets very very slightly separated from the surface upon mild heating. This generates an air gap between two layers.

Iridescence is a different phenomenon, which is angle dependent in the sense that you see different colors at different angles.

If you have over heated an empty stainless steel frying pan by mistake, that also forms a interference pattern (purple tinge). This is because of an oxide coating on the surface but it is not reversible. You have to scrub it away.

enter image description here

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  • $\begingroup$ I see, thank you for your clarification. However, this pattern on the ring does not change colour/shape when I look from it a different angle. Hence, is this observation still consistent with the proposed thin film interference theory? $\endgroup$
    – Brian
    Commented Apr 24, 2020 at 11:05
  • $\begingroup$ yes that is why this term iridescence is not correct in the context of your query. Thin film interference is more appropriate. $\endgroup$
    – ACR
    Commented Apr 24, 2020 at 11:34
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Elaborating on M. Farooq answer, if you have a surface covered in a thin transparent coating (oil, grease, oxide, et cetera), depending on the angle of incidence and wavelenght of light, the upper reflection may be either boosted or hampered by light coming from the lower reflection, so you see different colors depending on the angle. I did a lame drawing: enter image description here

This fact is used in powerful techniques like X-ray diffraction. See the wikipedia article on Bragg's law.

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