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Below, I show zooms on the glints on 3 materials: (clean) metal blade, stainless sink, artificial leather (but same effect shows on human skin). Only the sink was under artificial light. What I'm interested in is the colors appearing in some of these glints. Do you have an idea about what cause these colors ? (please mention whether your point is reasonnably sure or just a guess).

I do know pretty well the physics of light-matter interaction, but here it could be many thing I know (diffraction due to x or y, thin shell effect due to x or y or z, + possibly things I don't know. And I'm also a bit suspicious about the difference between theoretical materials vs real materials (with corrosion layer, traces of wetness or fat, possible extra treatment layers, mechanical and chemical aging, all might maybe cause thin shell effects or others causing colors...). My eye and camera can resolve the glinty details so I doubt diffraction is involved, but again I could miss some sub-aspect. enter image description here enter image description here enter image description here

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You say you (and the camera) are seeing glints from various surfaces; tiny flashes of light. Because these glints are bright, it means that you are seeing light coming from the surface - it could be reflected, refracted, diffracted, but the surface is providing light to your eye and the camera. You needn't be able to resolve the surface structures responsible in order to detect the light.

A.A. Michelson, Nobel Prize fin Physics for 1907, became interested in the bright glints from the carapaces of various beetles, and conducted experiments, and conjectured upon the ultimate causes.

Many appeared to be explicable in terms of diffraction, but the question was left open; we'll return to this momentarily.

The most common effect with metallic surfaces is interference due to thin coatings of oil, used to protect the surface. It is possible to remove all of the oil with proper solvents and ultrasonic cleaning, but the usual shop practices do not produce clean metallic surfaces. With interference through oil films you will see effects similar to oil on water: loops with the same color, and only a few colors being present, perhaps greens and reds. On a rough surface this will be hard to judge.

Diffraction effects require very small features, such as scratches. This is quite common on polished surfaces. To look for diffraction examine the very smoothest, cleanest surfaces under bright illumination. Let the returning light pass through a small aperture into a dark box - and fall onto a white surface. If there is a separation of the colors, you have diffraction.

Now we return to Michelson's conundrum: he found many cases that didn't seem to be interference or diffraction, and refraction was most unlikely. It wasn't until much later that it was found to be plasmons, quantized waves of charge, that provided the glints. Apparently beetles had discovered applications of quantum optics a hundred million years ago or so!

Here is an interesting article on the use of plasmonic glints in mythology and the middle ages: http://www.nature.com/nature/journal/v495/n7440_supp/full/495S8a.html?message-global=remove&WT.ec_id=NATURE-20130314

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    $\begingroup$ The blade of a pocket knife or kitchen knife is sharpened by grinding, which introduces fine parallel scratches of varying sizes due to the distribution of grains and grain sizes on the sharpening stone. That will produce random diffractions which will create the multicolored glints. If the surface was highly polished, the size of surface features will be too small to create anything other than a mirror-like reflection. $\endgroup$
    – Anthony X
    Feb 22, 2016 at 2:03
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    $\begingroup$ I've polished many surfaces too "mirror surface" when fabricating electron optics. When observed in bright light they were mirror-like, but under magnification you can always see fine scratches. Richard Feynman discusses this in "QED: the strange theory of light and matter" while discussing mirrors and diffraction gratings. $\endgroup$ Feb 22, 2016 at 2:18
  • $\begingroup$ Peter: I washed the knife with detergent, same for the stainless sink, so there shouldn't be any oil layer, right ? But might there is some other coating (permanent, like vernish) on the stainless ? And might the always occuring oxyd layer behave like thin-film ? Diffraction: I was surprise diffraction could occurs with scratch that does not seem to thin (since my camera resolve them). What do you call "color separation" ? view-dep colors ? (There is not enough energy to form an image on a screen, but I view-dep color can easily be seen). Plasmons: hum, any chance on ordinary surfaces ? :-) $\endgroup$ Feb 22, 2016 at 9:14
  • $\begingroup$ Anthony and Peter: well, ok mirrors are not "mirrors" when seen at near field (Hygens, etc), but isn't geometric optic supposed to be 100% valid when we are thousands time away of the scale of the largest bump ? $\endgroup$ Feb 22, 2016 at 9:17
  • $\begingroup$ After more observation, even a flat sheet of food aluminium has the behavior of my knife (but possibly one side is coated). And even a simple white plastic bag under the sun shows under careful observation viewdep (but colorless) micro-glints within the large specular spots ! But I know that some categories of glints are indeed taking place in the eye (because of heterogeneity of the cristalin). Is it the same for cameras ? $\endgroup$ Feb 22, 2016 at 9:22

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