# Sunglasses + Plane Window = Colors?

1. Jun 17, 2011

### gsingh2011

I was on a plane the other day and it was bright outside so I put on my sunglasses and looked outside and saw a spectrum of colors. Just looking through the window or just looking through the glasses didn't produce the colors. Any idea what's going on?

2. Jun 18, 2011

### turbo

I would guess that your sunglasses are polarized, and they showed you strains in the lexan window.

3. Jun 19, 2011

### gsingh2011

Well don't polarized glasses change the amount of light passing through based on the orientation? I tested it out and it seemed like the same amount of light now matter how I looked through them...

Also, why would polarized glasses show colors based on strains in the window? Unless the strains rotate the plane of the light there should be no difference.

4. Jun 20, 2011

### cragar

maybe the combined index of refraction was enough to break it up like a prism. Maybe

5. Jun 20, 2011

### Staff: Mentor

Only if you look at already polarized light. Try to look at reflections in glass, for best effects at about 55 degrees angle (best value is the exact Brewster angle, but anything around 45 should give visible results).

6. Jun 20, 2011

### sophiecentaur

That's it, exactly. The stresses in the glass make the electrons in the molecules more easy to vibrate in one direction than another (look up bi-refringence and polarisation). This means that the glass becomes a weak polariser of the light. So your polarising glasses will see this as varying brightness of the light coming through, depending on the actual amount of cancellation you are getting. The effect is wavelength sensitive so the various wavelengths are polarised in the glass at different angles. The result is that the partial cancellation of the light that you see is different over the visible spectrum - resulting in a slight colour cast which will vary as you rotate the glasses and look at different parts. The pattern of colour you see is due to the pattern of the different stresses. Toughened glass car windows show this effect very well sometimes because they are also toughened and a lot larger to show the effect more dramatically.
BTW, the 'polaroid' in sunglasses is made by stretching a sheet of the appropriate plastic to put stresses into it - a similar effect to that in your window glass.

7. Jun 20, 2011

### Andy Resnick

Turbo-1 has a good guess, but that would work only if the airplane window has a polarizer on 'the other side' of the window as well.

I don't know who makes the window assemblies, but I found this stuff:

http://www.epicos.com/epicos/extended/switzerland/mecaplex/products.htm [Broken]
http://www.docstoc.com/search/passenger-aircraft-cabin-window-construction [Broken]

Some people claim it's thin-film interference.

Last edited by a moderator: May 5, 2017
8. Jun 22, 2011

### gsingh2011

I'll give that a try.
That makes sense, but I saw all of the colors in all orientations, and I think I should only be seeing one at a time...
Why would you need another polarizer? And on my side of the window? That would be my glasses wouldn't it?
We only skimmed that in my physics class, but I highly doubt that.

9. Jun 22, 2011

### Andy Resnick

Stress birefringence (usually) needs to be viewed through crossed polarizers- one to allow polarized light to pass through the material and another to extinguish the unaffected light (this polarizer is sometimes called an 'analyzer').

In terms of thin-film effects, I agree interference is unlikely, but the film can act as a partial polarizer, for example via Brewster's angle. I've seen similar effects to what you describe when looking at (rear) car windows- I suspect the thin plastic films used as a safety device act as polarizers.

10. Jun 23, 2011

### gsingh2011

So in short what's going on is the stress in the glass polarizes the light once and the stress in the sunglasses polarizes it again causing the spectrum of colors.

Right?
It's cool seeing things I learned in school appear in the most unexpected places.

11. Jun 23, 2011

### Staff: Mentor

That's why there are taught.

12. Jun 23, 2011

### sophiecentaur

I'm trying to get to terms with what you say here. The patterns occur in the 'old' toughened glass windscreens which have no plastic. The pattern visibility does vary with conditions. I suspect that it is made visible when the incident light is, in fact, polarised. I seem to remember that sunlight may be polarised by the atmosphere (don't bees use this?) so that could satisfy your requirement for two polarisers. The effect is pretty subtle and the patterns are not very contrasty, which would be what you'd expect with only partial polarisation by the atmosphere.

@gsingh2011

We could clear this up if you could tell us whether the colours changed position as you rotated your glasses. (You would expect all colours to be there as the phase shifts are wavelength dependent and, as with thin film interference, you get selective cancellation). If not then I don't think this effect can be due to polarisation. Certainly the effect on car windscreens (colours and all) changes with rotation.

13. Jun 23, 2011

### Andy Resnick

In the US at least, automobile windows are both tempered and laminated since the 1920s, with plastic sheeting used as a safety feature to prevent glass from flying everywhere in case of an accident.

http://en.wikipedia.org/wiki/Laminated_glass

14. Jun 23, 2011

### sophiecentaur

In the UK I have seen many shattered windscreens (even one of my own), which shattered and you could push your fist through and drive on with good viz (no 'triplex' plastic layer until fairly recently - I AM old but not that old)). The same goes for side windows and you could get this effect in the right lighting conditions. So you can get the effect with just glass under stress. Here's your required polarisation:
http://www.polarization.com/sky/sky.html" [Broken]

Last edited by a moderator: May 5, 2017
15. Jul 4, 2011

### gsingh2011

They did. In fact, I took some videos and pictures while in flight (often when electronic devices were supposed to be off; aren't I rebellious :P). The taking these videos was often accompanied by akward stares from the other passengers... I'm back home now and won't be flying for a long time so I won't be able to get any more information.

The video links are below, I did my best using my phone and my sunglasses and I hope they give you an idea of how it looks. The three videos are essentially the same thing but one might show the effect better than the other, so watching all three would be a good idea.

There are also some images that I took that might be helpful (probably not as much as the videos). I attached them in a zip file. Also in the zip file, there are two images that caught my interest. One is looking at the tv monitor without sunglasses and the other is with sunglasses. This was a completely unexpected discovery since I was only focusing on the window, but it turns out the tv shows colors too. Anyone know why?