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Watching a video of a sheeve (pulley) being machined from steel I noticed a distinct rainbow-like pattern reflecting off the metal surface. This is visible at several different times in the video, here are some examples:

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(Source 1)

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(Source 2)

The steel has been heat treated prior to this machining step. As far as I could tell from the video the surface is very clean - cutting oil or other fluid was not used. Normally in machining processes the final surface is extremely smooth and free of defects. There could be very small tooling marks however, the cutting tools for turning on a lathe are often pointed cutters which are dragged across the new surface.

The visible pattern doesn't seem to fit the definition of refraction as in a prism where light passes through a surface between two media; and obviously it isn't "normal" specular reflection as I understand it.

I can't personally recall seeing this effect before and I've seen a fair share of machining videos... Of course lighting & photographic effects could serve to either amplify (in this case) or suppress (in other cases) so whether or not its really uncommon is hard to say from armchair experience.

I found this summary of some research where a type of refraction was deliberately created at a surface: "we've created the effects of a fun-house mirror on a flat plane". But that doesn't quite sound like the same visual effect.

So, what could cause this?

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1 Answer 1

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You have produced a reflection grating which you could change by changing the pitch of the cutting.

The cutter cuts the metal as the metal moves at right angles to the cutter an constant distance in a given time period - think of a screw thread being produced.
So you produce a series of equally spaced reflecting surfaces which in a similar way to having a series of equally spaced slits - diffraction grating.
The effect is due to the interference of light which is being reflected from the surface. The pattern can be used to judge the surface roughness of a machined surface.

You get the same effect with CDs and DVDs reflecting the light.

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  • $\begingroup$ I think your description of the machined surface is accurate. But its not clear why all those micro reflective surfaces combine to act in the same way as a grating (slits)? The slit experiment shows the light waves spreading out as they pass through. But specularly reflecting light doesn't do that, does it? If so wouldn't a normal mirror not function correctly? $\endgroup$ Apr 24, 2020 at 12:04
  • $\begingroup$ I don't intend to be obtuse, but I don't see how a solid reflection grating would behave the same as a diffraction grating (which has holes in it). Its not at all obvious why these different shapes would have a similar effect... The reference has good diagrams for diffraction but I didn't see one for a "reflection grating". I'm not disputing your answer itself, just looking for a deeper understanding. $\endgroup$ Apr 24, 2020 at 12:19
  • $\begingroup$ I imagine that an ideal surface of this type would look a bit like stair steps, with the surface normals of each 'run' of each step being parallel. So its an array of offset mirrors. This would certainly distort an overall reflection, but specifically why it acts like diffraction is harder to imagine. $\endgroup$ Apr 24, 2020 at 12:22
  • $\begingroup$ If the piece is made on a lathe type device a "thread" is produced which acts as a grating. Indeed that is how gratings are made. The interference pattern from surfaces is used to [check their smoothness]{zygo.com/?/met/profilers/opticalprofilersabout.htm}. $\endgroup$
    – Farcher
    Apr 24, 2020 at 12:29
  • $\begingroup$ Perhaps the best example of a "solid reflection grating" is a DVD/CD which produces a spectrum when viewed using incident white light. $\endgroup$
    – Farcher
    Apr 24, 2020 at 12:30

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