B Optical effect seen on a bus: mirrored screen shows different colors

Hello Physics People,

i was on bus 59 lately and i noticed a typical optical effect. There is a monitor/screen in the front of the bus that lists the current and coming stops and updates when you pass one. Its colour scheme is was blue/lightblue/white (see first picture). But when i looked at the screen via the window (so i saw its mirror image), its colour scheme was a nice rainbowy pastel scheme (see second picture). I believe it was raining outside, or wet, and the windows could have been wet outside. Maybe there was condens on the inside of the windows. My question: what's going on here? How does this happen? Does this effect have a name? I have no background in physics, only high school level.
Cheers, Thom

pictures: (sorry for the blurriness, the right was bumpy and my phone is not exactly the hubble space telescope, but only the colours are relevant anyway)
Screen in bus:
2yuc6s0.jpg

mirror image of said screen in side window:
34zbl3c.jpg
 

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tech99

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White light is composed of the colours of the rainbow. It is possible the raindrops were splitting the white light into its constituent colours, as we see with a rainbow. For this to happen, the angles must be exact, and we do not usually see it. The light is bent as it passes through a raindrop, but the blue end of the spectrum is bent more than the red, so the light is split up into its various colours.
Another way it can happen is if the glass has a thin film of oil or something like that on it. If the film is similar in thickness to the length of the light waves, the white light is split up into its colours.
 

DaveC426913

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Don't windows often have some sort of polarizing layer? I don;t quite know what it is , but you often see it on the rear windows of cars.

Sometimes it looks like a subtle rainbow of colours.

car-window-polarized-jpg.jpg
 

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sophiecentaur

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Don't windows often have some sort of polarizing layer?
Afaik, those patterns are due to the stresses in the glass when it is made. I think 'they' would rather it didn't happen.

34zbl3c-jpg.jpg

That picture is quite confusing to me, on the grounds that the stripes are so 'perfect'. Such perfection implies that there effect must be somehow generated in the display itself and not by random effects in the window glass or its coating. It would be interesting if the OP could look at the TV display off-axis and without the window reflection, in the same sort of direction that the image is being produced. I think it very likely that the same stripes might be seen.
Unfortunately, the 'experiment' would involve wandering around the inside of the bus like a loony and you could worry other passengers! Very risky to try to explain yourself to them. too! People suspect nerds. My local rural buses don't have that sort of display - just lumpy dot matrix.
 

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That picture is quite confusing to me, on the grounds that the stripes are so 'perfect'. Such perfection implies that there effect must be somehow generated in the display itself and not by random effects in the window glass or its coating.
Interesting point, this may be well the case. I shall check next time i am on the bus, as i take it regularly, maybe twice a month. And wandering around the bus is no problem. I'll take a picture of the screen. Besides, to enter the bus you have to pass underneath it. I'll also check if the effect occurs in dry weather (if it's not the screen)

Thanks all for your replies btw.
 
Having said that (see previous post), i have noticed this effect of recoloring of monitors too, when you look at them from an angle, but the angle needs to be very sharp, usually less than 10 degrees or so. And i was sitting in the middle of the bus, at perhaps 6 to 8 meters from the screen, in the seat next to the window. So the angle under which i saw it was much greater,and now i am doubting if this was what was going on. Anyway, i'll check next time.
 
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There could be a lot goings on here, or at least a lot of things to be considered...

1) the display looks to be LCD or LED - so the light can be polarized, and if color - each color may have a different polarization..
2) The safety glass - is a sandwich with plastic and is pre-stressed, to break in a specific way. But this causes some polarization in the film.

So a polarized source and a polarized glass (filter) - a reflection is a filtered image.

Next - while the photo may be close to what you saw, I suspect some of what we see is even "more distorted" than what you saw due to possible filter on the camera, and the rolling shutter effect - combined with the scan rate of the source....
 

sophiecentaur

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The safety glass - is a sandwich with plastic and is pre-stressed, to break in a specific way. But this causes some polarization in the film
That could explain blotches of colouration but the pattern in the picture is very regular and that implies it's part of a high quality bit of optics - or something to do with the leds or lcds - perhaps the angle varies over the height of the screen.
Of course, there is always the possibility of a graded filter on a sun visor somewhere up there. The OP just has to get on a few more buses and see if the same phenomenon relates to the route of the particular vehicle.
 

DaveC426913

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Afaik, those patterns are due to the stresses in the glass when it is made. I think 'they' would rather it didn't happen.
Totally.

View attachment 235870
That picture is quite confusing to me, on the grounds that the stripes are so 'perfect'.
??
monitor.jpg


I think what's throwing you off is that the table of data has its own even-odd colour banding, which makes it look like the rainbow colours are all parallel and horizontal.
 

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sophiecentaur

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I think what's throwing you off is that the table of data has its own even-odd colour banding, which makes it look like the rainbow colours are all parallel and horizontal.
That's true but still, it looks to me that the effect - interference related - would require very little change in optical thickness over a big area. When you see the patterns in a windscreen, there are many max/min cycles over the area and oil film patterns on water are much more variable and tighter. That was why I was looking for a very uniform thickness in whatever could be causing such an interference pattern. Perhaps production methods for 'regular' triplex type glass are better than I imagined.
One thing that would be of interest would be how those patterns change or move as the observer moves up and down.
 
So i went on this bus again the other day, and the 'rainbow'effect did not occur. I have included two pics, the top one showing the mirror image (with the same blue colors as the original) , and the bottom one showing the actual monitor. The monitor itself does not show very different coloring, when viewed from a sharp side angle ( i investigated this on the bus). The weather and the windows were dry. I asked the bus driver if the bus had some special coating on the windows, and he answered that he was not aware of any, if any. He thought there wasn't . A different type of bus used by this company does have a sort of coating on the window, but that type of bus is not used on this specific line. So now i think it does have to do with the water drops on the windows, or perhaps the monitor in my first post was broken in some way.
20181228-175844.jpg

20181228-175849.jpg
 

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126
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Anti-graffiti window film in the first bus seems to me a plausible explanation. Compare the photo below, which was taken in a train with such window film, and a polarizing filter on the camera, at night. The colored fringes in the reflection are due to birefringence of the polyester film. The film is on the inside of the window, so light rays that are reflected by the glass surfaces of the window pass twice through the film. Although your situation was different, as you had no polarizing filter on the camera, the polarized light from the LCD screen would result in similar colored fringes.
treinraam.jpg


If you happen to have polarizing sunglasses, next time in the bus you might have a look through them, to examine if the LCD screen is polarized, and if the colored fringes extend beyond the borders of the LCD screen.
 

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sophiecentaur

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Anti-graffiti window film in the first bus seems to me a plausible explanation. Compare the photo below, which was taken in a train with such window film, and a polarizing filter on the camera, at night. The colored fringes in the reflection are due to birefringence of the polyester film. The film is on the inside of the window, so light rays that are reflected by the glass surfaces of the window pass twice through the film. Although your situation was different, as you had no polarizing filter on the camera, the polarized light from the LCD screen would result in similar colored fringes.
View attachment 236623

If you happen to have polarizing sunglasses, next time in the bus you might have a look through them, to examine if the LCD screen is polarized, and if the colored fringes extend beyond the borders of the LCD screen.
That photo is the sort of thing I'd expect - several wavelengths of slope across the film and a random looking position for the centre. The original images are still a surprise to me. I will keep my eyes open when I use buses in future.
 

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