Colour patterns from the reflection of my window

Question

The first picture is that of my window while the second is its reflection as seen from the floor. The pattern depends on the angle. That is when I move a little closer then look at the reflection the left part of window becomes transparent and the pattern is only seen at the middle and the end.

What causes this?

I don't think it can be thin-film interference from any thin layer formed at the surface of my floor since the pattern is uniform while the floor cannot be so smooth.

Also if this was caused by dispersion then the white light from my led should also get dispersed and I don't think that the dispersion would depend on the angle. These are my attempts to understand this. Any input is appreciated. Some new observations: 1)The pattern is only visible when the window is closed. If I keep a mirror on the floor where the pattern is formed , the reflection from mirror causes no pattern while the rest shows the same pattern.

2)The pattern is most prominent at the evening when the Sun is almost below the horizon (sunset). I have completely ruled out thin-film interference with this new data.

My best guess now is that my window mirror absorbs some colors and this is enhanced by the reflection from the floor.

Related to Alphy's answer

How does the thicker thinner argument reconcile with the uniform colours and almost constant widths of each colour at a slanted angle? Also the colours get repeated after some interval which would imply that thickness decreases then increases or that the angle somehow (still confused about the angle argument) to give thaf red which seems unlikely.
You have mentioned that reflected light enhances the effect (which I also think) but putting a mirror where image is formed does not give any colour. So why would reflection from floor only do this and not the mirror? 3)"The glass changes the angle (and it does so differently for different wave-lengths) at different points, therefore, light gets censored differently at different points" yet the colours have a width and are uniform. This would imply that the change in angle at different points etc etc would have to happen in a very special way to give this pattern.

enter image description here

What kind of data? Plz elaborate I am sorry I do not understand. — Lost, Sep 24 '20 at 17:03
photos of the phenomenon (aka: data), with a polarized filter in various orientations. — JEB, Sep 24 '20 at 22:30
The angle not but the "linear" separation would certainly do. I think you need some for your eyes to be observed. — Alchimista, Sep 27 '20 at 12:40
I won't be able to take the data using a polarization filter. Though I collected some other data. — Lost, Sep 27 '20 at 12:52
The pattern is only visible when the window is closed. If I keep a mirror on the floor where the pattern is formed , the reflection from mirror causes no pattern while the rest shows the same pattern. The pattern is most prominent at the evening when the Sun is almost below the horizon (sunset). I have completely ruled out thin-film interference with this new data. My best guess now is that my window mirror absorbs some colors and this is enhanced by the reflection from the floor. — Lost, Sep 27 '20 at 12:58
Is your window glass covered by a plastic film? A birefringent plastic film would explain the color pattern. — jkien, Oct 13 '20 at 09:08
I agree with JEB that this phenomenon is likely related to polarization, so it would be great if you could take a photo through some kind of polarizing filter. Even an old pair of Polaroid sunglasses would be ok. You can buy small linear polarizer sheets for a few dollars. There are tons of them online, or try a local camera shop or optometrist / optician. Another option is old 3D glasses as used in movie theatres, although they're likely to use circular polarizers, not linear ones. — PM 2Ring, Oct 13 '20 at 14:28
I have certain queries regarding your answer Ill add them as an edit to my question since comment space is less. — Lost, Oct 14 '20 at 18:17
@Lost I have given an answer to your queries in an edit to my previous answer. Let me know what you think about them. — Alphy, Oct 14 '20 at 19:06
Is what you see in the reflection simply the sky? I.e. is the building small enough that it's invisible in the reflection, and only the sky is seen? Also, is the window the usual glass window, or is it maybe plastic, quartz or something else? — Ruslan, Oct 15 '20 at 13:54
Hi Ruslan. I have put up an image with an open window. The house is small so we can directly see the sky. I am not sure about the material with which my window is made but it surely isn't normal glass since it tones down or decreases the intensity of the light. I think it has polaroid film. Though I am not sure. Also I am pretty sure the phenomenon is happening because of the "window" and the "floor" since if I open a window the pattern disappears on the other hand the pattern on the 2nd window darkens as now there are two windows (these are slider windows so u slide one behind other to open) — Lost, Oct 16 '20 at 11:14

Alphy · Accepted Answer · 2020-10-14T19:05:36.530

1

What is happening, in my opinion, is the result of two effects:

First one is to notice that the glass of your window is most probably not uniform. Meaning, as a guess, it is thinner on the top and gradually gets thicker to the bottom, but in a very slight manner that only light feels it. What does this cause? Well if the glass also has two different refraction indices, say $n_v$ for vertical refraction index and $n_h$ for horizontal refraction index, then the polarization of the light traveling through the glass will rotate due to this non-isometry $n_h \neq n_v$. But remember that the glass at some point is thicker than the others; which leads to the fact that the polarization rotates more at those points that are thicker.

The second is to note that the incoming light is probably polarized to some extent since, from the looks of it, it is reflected from the wall in front of your window. In general reflected light gets polarized and this effect is more intense for certain reflection angles. So you have a polarized light which rotates differently at different points and then gets one more time reflected from the ground. This second reflection also polarizes the light one more time and cuts out those lights that are opposing its direction of polarization.

Therefore, essentially, some of the reflected light from the ground gets censored (by the procedure of polarization). Which light is censored depends on the angle of polarization of the light. The glass changes the angle (and it does so differently for different wave-lengths) at different points, therefore, light gets censored differently at different points and for different colors; leading to the pattern you are seeing.

Now you might ask:

How does the thicker thinner argument reconcile with the uniform colours and almost constant widths of each colour at a slanted angle? Also the colours get repeated after some interval which would imply that thickness decreases then increases or that the angle somehow (still confused about the angle argument) to give that red which seems unlikely.
You have mentioned that reflected light enhances the effect (which I also think) but putting a mirror where image is formed does not give any colour. So why would reflection from floor only do this and not the mirror?
"The glass changes the angle (and it does so differently for different wave-lengths) at different points, therefore, light gets censored differently at different points" yet the colours have a width and are uniform. This would imply that the change in angle at different points etc etc would have to happen in a very special way to give this pattern.

To answer these queries it is simpler to imaging that the incident light includes only one frequency (let's say red) of the whole visible spectrum.

If the thickness of the glass gradually increases from top to bottom, then our red polarized light gradually rotates from top to bottom. Of course, the transformation of rotation is periodic, therefore after a while the direction of polarization comes back to its initial state. This then leads to having an image constructed by ordered bands of dark and light red.
The reflected light is polarized to some extent but there is an optimum angle for polarization that is different in different material. I suspect that if you look at the mirror while holding it with a different angle than the floor (probably almost perpendicular to the incident light) then you might be able to see the same phenomenon.
It is kind of a special way, since it is periodic.

edited Oct 14 '20 at 19:05

answered Oct 12 '20 at 21:40

Alphy

353

1

I have added my queries regarding your answer to my question. – Lost Oct 14 '20 at 18:31
I will try this out with the mirror at different angles before proceeding to discuss further. The pattern usually is almost non-visible at raint days so Ill try it at a suitable time. Also, is the optimum angle you are talking about Brewster's angle?

Lost

Oct 15 '20 at 09:49

@Lost Yes I am talking about Brewster's angle. – Alphy Oct 15 '20 at 12:56

Hi Alphy. I have put an image with the mirror. Check it out. This is an old one and I havent got the chance to try it at different angles due to the weather. I will post it as soon as I can. – Lost Oct 16 '20 at 11:35

In any case. The mechanism givem by you seems correct as the contribution of polarizationa from both window and mirror seems to be causing it. I have to study how some colours completely get removed (absorbed I guess) at different due to this rotation in polarization. – Lost Oct 16 '20 at 11:39

Hi @Lost Thanks for the picture with the mirror but I don't think you can see any thing with that angle (parallel to the floor). For such effect to happen you need two step polarizations one from the floor and the other one can be either from the wall or the sky, since there is also an optimum polarization angle of reflection for the sky. – Alphy Oct 16 '20 at 12:34

@Lost let me give you another experiment to perform. Use some transparent Scotch tapes (glue tapes) and tape them on the glass with different angles. Let me know what happens to the pattern on the floor. You can also tape different transparent material such as a transparent plastic spoon or ruler. If my answer is correct the pattern most probably should change on those areas. – Alphy Oct 16 '20 at 12:44

You make the assumption that the glass has two different indices of refraction. What do you base this on? – Deschele Schilder Oct 16 '20 at 12:52

@DescheleSchilder This happens is some material due to different reasons such as an anisometric stress or noncubic crystal structure. Look for "birefringence". – Alphy Oct 16 '20 at 13:24

But why do you assume the glass in the window has this property? – Deschele Schilder Oct 16 '20 at 13:35

Hi Alpy I will try the tape experiment. On that note , what do you mean when you say that reflection is needed from a wall or something elese etc. Since originally the pattern is forming because of the "window and reflection from the floor". – Lost Oct 16 '20 at 14:09

@DescheleSchilder I am guessing such things can happen irregularly it the production process or installation? Also the glass might be old and over time because of the gravitation such anisometry might be produced? There are multiple possible ways. – Alphy Oct 16 '20 at 15:12

@Lost Where does the light going through the glass is coming from? Is it directly from the sun? Or it is reflected back from the wall infront of the window or the sky? For my answer to work a 2-step polarization is needed, one is the floor the other, as per my guess, is either the wall or the sky. – Alphy Oct 16 '20 at 15:14

@Alphy I think the glass doesn't have an isometry as far as the indices of refraction are concerned but if so, then the light rays will go through the glass in the same direction as the direction in which they fall on the window. If light falls on a glass plate, then the ray will continue its way in the same way as it felt on the window. The differences in path lengths do not cause observable interference, as the glass plate is too thick. Only a thin film (like the one on the floor) will cause observable angular dependent interference. – Deschele Schilder Oct 16 '20 at 15:57

@Alphy: There are two things: 1) Be it the sky or the wall if the window is open the reflection from the floor is not producing the pattern. So 2) The possibility is that the 2-step polarization is happening cause of the "window glass then the floor". – Lost Oct 17 '20 at 14:53

jkien · Answer 2 · 2020-10-15T16:17:46.433

1

Your colour effect is somewhat similar to what can be seen through the windows of some trains. A part of the blue sky is a source of polarised light. When viewing that part of the sky through a birefringent window and a linear polarisation analyser, it is possible to see the colour pattern. It is more likely (and cheaper) a window is birefringent due to a polymer plastic film, than due to the glass.

The sky seen through a train window, with a linear polariser on the camera. There is no film in the corner.

edited Oct 15 '20 at 16:17

answered Oct 15 '20 at 16:12

jkien

2,564

It could be this, yes. Could it he that the floor is acting as a linear polarizer? Like your analyser. Also what do you mean by "some part of the sky is a source of polarized light?". Also if this were the case then could a reflection from a plabe murror instead of the floor give the same pattern?. Lastly, the mechanism of this phenomenon is essentially what Alphy pointed out isn't it? – Lost Oct 16 '20 at 11:18
Reflections from floors tend to be strongly polarized; a search term is “Brewster’s Angle.” The strong polarization of reflections from horizontal surfaces is why driving is more comfortable with vertically polarizing sunglasses. – rob Oct 16 '20 at 11:24
Yes I am familiar with Brewster's angle but that would imply that even a plane mirror should produce the same effect at a particular angle as lointed out by Alphy , ryt? – Lost Oct 16 '20 at 11:33
Do you think a film is placed on the window in the OP's room? Why is the pattern seen without a linear polariser? And why isn't the pattern seen in the mirror? Doesn't the last fact imply that the floor is responsible for the pattern? – Deschele Schilder Oct 16 '20 at 12:28
Yes. The last fact implies floor is responsible but together with the window. – Lost Oct 16 '20 at 14:05
@Lost. Yes, the floor is acting as a linear polariser. In contrast, the aluminium surface of a mirror does not polarise light, so when using the mirror, the colour pattern does not appear. The maximum polarisation of the blue sky is found at ninety degrees from the sun. So when the sun sets in the west, the polarised sky stretches from the north to the south. Yes, my explanation is basically the same as the explanation by Alphy. The only difference is whether the birefringence of the window is caused by the glass or by a plastic film on the glass. – jkien Oct 16 '20 at 22:06
Another difference is that I assume the thickness of the window is uniform (not variable). The different fringes occur because different light rays converging towards the eye are not parallel. In contrast, Alphy proposes a variable thickness. Perhaps Alphy is assuming that different light rays converging towards the eye are parallel. – jkien Oct 16 '20 at 22:42

Deschele Schilder · Answer 3 · 2020-10-30T21:17:10.047

0

Thanks for making (and showing) extra pictures.
In the second extra picture, I can see faintly the described effect in the reflection by the floor but not in the reflection by the mirror. Just look on your screen at a small angle from below, which enhances the effect. So even if the window is open, the effect is visible, so we can eliminate the effect of the window. In the first picture (is it taken on a cloudy day?), no pattern is visible indeed. But you took the picture much closer to the window (so changing the angle). As the effect I describe is angle-dependent this could be possible (you write: That is when I move a little closer then look at the reflection the left part of the window becomes transparent and the pattern is only seen in the middle and the end).

Plate glass has only one refractive index (independent of the orientation see here: 1.52). Its thickness may vary but this gives no visible interference effects because of the thickness of the plate.

So we are left with only one possibility: The floor is responsible. If you can see this effect with the naked eye, the cause can be the same as the cause of rainbow patterns of oil on water. But since you ruled out this cause because of the roughness of the floor without a layer, the cause must be due to the effect of the floor on all polarizations.
This means that all polarizations are reflected in slightly different ways (just as in the water-oil case).

edited Oct 30 '20 at 21:17

answered Oct 15 '20 at 00:07

Deschele Schilder

1

It's not from the floor since not all lights result in the same effect. For example the room's light bulbs are not producing such an effect. – Alphy Oct 15 '20 at 02:25
It cannot be thin film inteference because the pattern is very smooth and my tile floor would have to be smooth on the levels of nm which is impractical. Also if I open the windows , then no pattern is formed on the floor. The interference shouldve happened irrespective whether the window was open or not but thats not the case. Thats whu I have already ruled out such interference assumptiins – Lost Oct 15 '20 at 09:38
I made an edit and explain why the light in her room doesn't show the effect. – Deschele Schilder Oct 15 '20 at 10:35
@Lost The last comment isn't meant for you (well, of course, it is, but not for your last comment). What do you mean that the tile floor would have to be smooth on the level of nm? That the thickness of the film varies? Well, it almost certainly varies, but the thickness seems not to vary that much to make the interference go away (you can even say that this proves the thickness of the film varies barely; the film is formed from a liquid that gets dried). As far as the open window is concerned. When the window is opened more outside light reaches the floor, which can make the pattern invisible – Deschele Schilder Oct 15 '20 at 10:59
@Lost Why should the pattern disappear in the mirror, while it stays visible on the floor? – Deschele Schilder Oct 15 '20 at 11:02
@Lost Or do you mean that the floor without the film has to be smooth? – Deschele Schilder Oct 15 '20 at 11:04
@Lost Can you take a picture of the floor with the windows open? Maybe when the Sun is low on the horizon (at Sunset)? Does the Sunlight fall on your floor at that time? – Deschele Schilder Oct 15 '20 at 11:13
Yes. I have I will post it. The sunlight falls with the windows open but there is no pattern – Lost Oct 16 '20 at 11:02
What I mean by smoothness is that the variation in the film or the floor has to be smooth in nm. For instance , if there was a variation of even 50nm which there probably is then the there would orange mixed with red. – Lost Oct 16 '20 at 11:04
Also I am pretty sure that the effect is due to both "mirror and floor" as you can see in the latest pic I put up in my question. – Lost Oct 16 '20 at 11:19
@DescheleSchilder The fact that LED's spectrum is that way does not prevent the color patterns to form. The color patters should form even if there is only one color which leads to light and dark bands of that color. Also not all room lights are from LEDs. So I don't think your answer here is relevant. – Alphy Oct 16 '20 at 12:38
If the LED would emit one wavelength, the constructive and destructive interference (dark and light bands of one color) would be too close to each other to observe them and you'll see a uniform color without dark and light bands. – Deschele Schilder Oct 16 '20 at 12:51

Colour patterns from the reflection of my window

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