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Lately, I observed a rainbow-like ring forming in my window during nighttime when the light enters from a street lamp. Interestingly, the sequence of colors in the rainbow seems to be reversed from what I anticipated. Normally, I would expect the inner part to be red and the outer part to be blue due to the greater refraction of blue light (like a sun halo). What could be the reason behind this color reversal phenomenon?

rainbow in window

Edit: To clarify, the window has a decorative film on it which causes the rainbow. The core question is still the same: Why would blue be on the inside and red on the outside?

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  • $\begingroup$ This is expected behaviour: Usually the Sun is behind you, and now the Sun is in front of you. You are looking at a different number of reflections, quite like the 2ndary bow, which is also colour-reversed from the usual. en.wikipedia.org/wiki/Rainbow $\endgroup$
    – naturallyInconsistent
    Commented Jul 13, 2023 at 3:14
  • $\begingroup$ Could you have a look if your window is covered by a decorative film with a rainbow effect, like this one: youtube $\endgroup$
    – jkien
    Commented Jul 13, 2023 at 8:58
  • $\begingroup$ I forgot to mention, it is covered with a decorative film which causes the rainbow. $\endgroup$
    – Brad Thiessen
    Commented Jul 13, 2023 at 13:23
  • $\begingroup$ @naturallyInconsistent I understand the reasons behind the color inversion of the secondary rainbow. However, in this instance, I'd expect the color arrangement to resemble that of a sun halo, with red on the inside and blue on the outside, considering the light source is positioned at the front. $\endgroup$
    – Brad Thiessen
    Commented Jul 13, 2023 at 13:42
  • $\begingroup$ I suspect this is related to thin-film interference or diffraction gratings, and not to refraction (en.wikipedia.org/wiki/Thin-film_interference), but I'm struggling to come up with a complete explanation of what's happening here, and online sellers don't say how it works. I suggest changing the phrasing of your question from "why is it blue on the inside" to "why is this happening." Given that we don't understand why this is happening, it seems unwise to me to have any expectation of what the order of the colors should be. Especially since there's also blue on the outermost edge. $\endgroup$
    – AXensen
    Commented Jul 13, 2023 at 14:13

2 Answers 2

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The color sequence on your photo is, from the center towards the periphery: 1st order spectrum blue - green - yellow - red; and then the 2nd order spectrum beginning with blue. So each triangular piece (or polygon) of the window film behaves like an ordinary grating.

enter image description here

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  • $\begingroup$ Could you elaborate on what you mean by each film behaving like an ordinary grating? Any chance you have a diagram showing what's going on? $\endgroup$
    – Brad Thiessen
    Commented Jul 13, 2023 at 16:41
  • $\begingroup$ A grating or hologram can be "printed" on a plastic film by an "embossing machine", like this one. Your film is a mosaic of triangles (or polygons). In your photo, each triangle behaves like a 1-dimensional grating. The direction of the grating lines within the triangle can be radial with respect to the center, where the lamp is seen, or transverse, or a random direction in between. In the colored triangles of your photo, the direction of the grating lines is transverse. $\endgroup$
    – jkien
    Commented Jul 13, 2023 at 22:37
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There are two possible explanations for what you are seeing:

First, the fundamentals:

Prism by Edmund Optics

A prism is an optical device that, through normal dispersion of glass (that means, $n(\lambda)$, where $n$ is the index of refraction of glass and $\lambda$ is the wavelength of light, separates the components of white light due to the different optical paths that each wavelength travels through the material, resulting in the spectrum becoming clearly visible at the output window of the prism.

For your window, it is important to know a few things:

  • Is the window really made of cut glass, or is that a film (as others pointed out) that has tinted sections? I personally think this is not the case, as the pattern looks rather structured (for rainbow films I have seen that the colors are more unevenly distributed).
  • Does this only happen at night? Is the moonlight directly hitting the window?
  • Does this also happen with sunlight?

The last 2 questions I don't think they are too important, it is just to understand better how the window itself is built.

Based on the picture only, I would assume that the cut glass is somehow circular in design (as it happens when, for example, in nature, the shape of raindrops is roughly spherical leading to the circular rainbow pattern). Therefore, I would broadly speculate that the cut glass is somehow thicker at the center causing the normal dispersion in glass, or maybe the light source plays a role.

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  • $\begingroup$ "assume that the cut glass is somehow circular in design" — I think the circular pattern is not due to the design of the glass, but rather due to the geometry of scattering, similar to how random scratches create a circular pattern on reflection of bright light, as in this photo. (This particular phenomenon with scratches is discussed in this thread.) $\endgroup$
    – Ruslan
    Commented Jul 13, 2023 at 12:53
  • $\begingroup$ I forgot to mention, there's a film over the window which causes the rainbow. Either way, I would expect the colours to be reversed. $\endgroup$
    – Brad Thiessen
    Commented Jul 13, 2023 at 13:21
  • $\begingroup$ To answer your question, it happens with all light but it's easier to see at night. The light in the picture is a street lamp. $\endgroup$
    – Brad Thiessen
    Commented Jul 13, 2023 at 13:23
  • $\begingroup$ In my opinion that glass looks way too flat and thin to be producing such a dramatic rainbow from prism/refraction effects. I think some kind of thin-film interference effect is more likely but from the details here I cant really figure out what exactly it is. $\endgroup$
    – AXensen
    Commented Jul 13, 2023 at 14:08

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