Holed up at home, I was distracting myself by shining my green laser pointer around. Something particularly interesting happened when I pointed the laser at a reflective black plastic surface. The reflected beam would show first as a dot, but rapidly start to spread out and achieve a sort of halo shape. Waiting longer yielded a sequence of concentric almost-rings, and reminded me a lot of the diffraction patterns I learned about in undergrad physics.

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I assume the laser is actually heating and warping the plastic here, but I'd love to hear a more detailed explanation if someone has one.

Edit: Yesterday I set up the laser to point at a spot on the surface, which is the casing of a fan. As you can see in this picture, it actually altered the surface after several minutes. It didn't feel warm at all, but it seems clear to me that there's some heat-driven effect.

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  • $\begingroup$ I have witnessed a similar phenomenon before, but I have always been baffled about it. I'm quite sure that the laser spot changes something in the plastic (chemically or physically). If you want to take the time, maybe you can make a video, including moving the piece of plastic once? $\endgroup$ – zonksoft Apr 19 at 19:13
  • $\begingroup$ Can we also see a photo of the black plastic surface? $\endgroup$ – boyfarrell Apr 20 at 14:52
  • $\begingroup$ @boyfarrell see edit $\endgroup$ – Ethan Dlugie Apr 21 at 0:32
  • $\begingroup$ You could try moving it to another spot to watch the ring form in real time. Try and make a time lapse recording, it very slow. This would tell you if it is sure to deformation of the surface. I notice the surface is also slightly curved. This could cause thing ring effect too. Try and eliminate one of these possibilities $\endgroup$ – boyfarrell Apr 21 at 7:34
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    $\begingroup$ Ok, so the plastic is visibly heated up within seconds. That must be some laser pointer! $\endgroup$ – zonksoft Apr 29 at 17:44

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