I placed a sheet of glass (8 mm thick) upon opaque, non-glossy paperboard. The paperboard has a pinhole with a bright light source behind it.

arrangement drawing

Here's a photo as well:

arrangement photo

And here's what it looks like if you don't look straight down:

photo of the experiment

As you can see, there's 28-mm wide spot of more or less uniform brightness. Also, not visible in this photo, there's much dimmer and larger (56 mm) spot with the same sharp boundary. The brightest spot, which is less than 10 mm, has maxed out on camera and actually looks more like a glowing ring around the light source. Below is the output of 3D raytracing renderer, to give you an idea of what that ring looks like:

Concentric spotlights formed by internal reflections inside the glass screen

Thickness of the glass and size of 28-mm spot suggests that the effect is related to the phenomenon of total internal reflection, though I fail to understand how exactly. Is there a model for these spotlights, which is simpler than doing the raytracing with Fresnel equations?

  • $\begingroup$ Where the two concentric spotlights coloured in any way? $\endgroup$
    – Farcher
    Nov 24, 2022 at 9:43
  • $\begingroup$ @Farcher they looked white. If there were any rainbows, I didn't notice them. $\endgroup$ Nov 24, 2022 at 9:45
  • $\begingroup$ I have not been able to reproduce the effect that you see. Is it possible fro you to provide a drawing of the arrangement and what you have observed? $\endgroup$
    – Farcher
    Nov 24, 2022 at 10:02
  • $\begingroup$ @Farcher I made a better pinhole, so I could actually make a photo. Also, found mistake in my previous measurements. Updated the post. $\endgroup$ Nov 24, 2022 at 17:34


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