The discussion revolves around the phenomenon of seeing a spectrum of colors when looking through sunglasses at an airplane window. Participants explore the underlying physics, including the roles of polarization, stress birefringence, and potential thin-film interference, while considering the conditions that lead to this visual effect.
Participants do not reach a consensus on the exact cause of the observed colors. Multiple competing views regarding the roles of polarization, stress birefringence, and thin-film effects remain present throughout the discussion.
Some participants note that the visibility of patterns in toughened glass may vary with conditions, and there is uncertainty about the polarization of sunlight by the atmosphere. The discussion also highlights the complexity of the interactions between light and materials, which may not be fully resolved.
turbo-1 said:I would guess that your sunglasses are polarized, and they showed you strains in the lexan window.
gsingh2011 said: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...
gsingh2011 said: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.
gsingh2011 said: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?
turbo-1 said:I would guess that your sunglasses are polarized, and they showed you strains in the lexan window.
I'll give that a try.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).
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...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.
Why would you need another polarizer? And on my side of the window? That would be my glasses wouldn't it?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.
We only skimmed that in my physics class, but I highly doubt that.Some people claim it's thin-film interference.
gsingh2011 said: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.
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').
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.
gsingh2011 said:It's cool seeing things I learned in school appear in the most unexpected places.
Andy Resnick said: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.
sophiecentaur said: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.
sophiecentaur said:@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.