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I'm wearing glasses with a sunglass clip-on. This means I have my regular glasses and, on top of them, I have a second pair of lenses that work as sunglasses and attach to my regular glasses using tiny magnets.

I'm also in a car, and I'm looking through a window layered with Insulfilm. Curiously, when I look through the Insulfilm with the sunglasses on, I see a rainbow pattern. If I keep my glasses on, but remove the sunglass clip-on, the rainbow is gone. If I look through a window without Insulfilm, the rainbow also seems to be gone. If I look through the Insulfilm with the sunglasses clip-on, but without my regular glasses, the rainbow is there.

I believe my sunglasses are polarized, and the rainbow fades away when I tilt my head a little.

Why exactly is this phenomenon happening? I guess it has something to do with polarization due to the rainbow fading when I tilt my head, but I can't figure out why it is a rainbow and why I can't see it without the sunglasses.

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    $\begingroup$ I would guess a photoelasticity effect: en.wikipedia.org/wiki/Photoelasticity. I see something similar with my polarized clip-on sunglasses and my car windows. Maybe try looking at a stressed colorless plastic item, like the CD or DVD case in the link, with your sunglasses? Cellophane works very well. $\endgroup$
    – Ed V
    May 6, 2023 at 20:35
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    $\begingroup$ @EdV I tried a plastic ruler and I can indeed see rainbow patterns, while I would see none without the sunglasses $\endgroup$ May 6, 2023 at 23:42
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    $\begingroup$ Good! So my hunch was not bad. If you gently bend the plastic ruler, while observing it though your sunglasses, the colors will shift and alter. Glassblowers use a variation on this to detect strains in newly fabricated pieces (prior to annealing the piece) and, in the old days, engineers could use the effect in modeling. I already upvoted your question and will do likewise for the one answer available. $\endgroup$
    – Ed V
    May 6, 2023 at 23:59
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    $\begingroup$ And I should have checked my hunch, given how much is already at this stack exchange: physics.stackexchange.com/a/172674/313612. ;-) $\endgroup$
    – Ed V
    May 7, 2023 at 0:14

2 Answers 2

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Trying not to introduce math:

Yes, it looks like your sunglasses are polarized.

What you are seeing is one of the possible behaviours of a birefringent material called photoelasticity. Essentially, birefringence means that the material interacts with light in a way that depends on its orientation. This is something that often happens in crystals that are very ordered structures, or polymer materials (materials made of long molecules). Insulfilm is likely mostly made of polymers. And films are usually made by stretching or pressing an initial block of material until it makes a film of the right thickness. After this process, the material tends to be stressed, and polymer chains tend to align in a particular direction. This introduces the orientation-dependent behaviour at the molecular level that leads to birefringence because light interacts with the molecules as it passes through the material.

Now, why can you see the effect only through polarizing glasses and not your naked eyes or through regular glasses? The reason is that, to see the colorful-kind of birefringent effects, you need light that has itself orientation-dependent properties (I mean more that just the orientation introduced by its direction of propagation). In fact, the usual setup to see the rainbows in birefrigent materials is to place the material between two polarizers. You already wear one, and light that arrives on the film is probably already partly polarized (for example by the sky). With a second polarizer you would be able to make the rainbow pattern more contrasted.

You can have a look at https://en.wikipedia.org/wiki/Birefringence for a broad overview of birefringence, and more specifically photoelasticity in that same page.

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    $\begingroup$ Reflection can give polarised light, which is why polarised sunglasses can help to reduce reflection glare. See en.wikipedia.org/wiki/Brewster's_angle FWIW, the maximum polarisation of the sky light is at 90° from the Sun. So if you look at the Moon through a polariser near first or last quarter phase it can greatly improve the contrast. $\endgroup$
    – PM 2Ring
    May 7, 2023 at 3:11
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I agree with Roan's answer that the rainbow colors are caused by your insulfilm being birefringent, using polarizing sunglasses, and the light source emitting polarized light.

I would like to add that the polymer film is probably biaxially birefringent, and that you could verify this simply by moving your eye closer to the film. You should then see the bullseyes pattern. The two bullseyes represent the axes of birefringence. Rotating your sunglasses should have the effect illustrated below. By rotating the sunglasses 90° you should get the same pattern in complementary colors. This is why you don't see the color pattern without the polarizing sunglasses: the addition of complementary colors results in white light.

enter image description here

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