Naked Science Forum
General Science => Question of the Week => Topic started by: techmind on 23/11/2007 15:59:04
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Techmind beat the Naked Scientists to this one, so to see the original question and the answer given on the show, scroll down
In the past two weeks' radio programmes, a question from Lauren has been aired asking why we see rainbow-colours in things while wearing polarizing sunglasses.
Here's my answer:
Light can be polarised (or partly-polarised) by reflection e.g. from the surface of glass or water.
This effect is strongest when the angle of reflection is close to the Brewster Angle (Wikipedia it!).
By arranging their axis of polarization perpendicular to the light scattered/reflected off horizontal surfaces, polarising sunglasses can dramatically cut down the glare of light reflected from horizontal surfaces such as the sea or road - while only reducing the brightness of everything else by about 50%.
If you put two polarizers in the path of light, with their axes crossed, then you can block the light completely.
However, many plastics can rotate the polarization of light, and typically rotate different wavelengths (colours) by different amounts. The amount of rotation can also be affected by applied or residual stresses in the plastic or glass. Consequently if you put a piece of plastic (or pre-stressed glass - glass that shatters into thousands of tiny pieces when you break it) between crossed polarizers you can see lots of pretty colours.
See my Kitchen Science experiment:
http://www.thenakedscientists.com/forum/index.php?topic=6730.msg129590#msg129590
Since the ambient light on a clear day can be partially polarised from the sky or other reflections you can sometimes see weaker versions of these effects even without deliberately/explicitly polarizing the light to start with.
In certain scenes the light can be partially polarised, and for various reasons the predominant polarisation may vary with wavelength (colour). Usually we don't notice these effects because our eyes are not polarisation-sensitve. When you wear polarising glasses however the polaroid material "analyses" the polarisation, converting polarisation effects to intensity effects... and revealing hidden colours!
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Oh, now that's not fair...i was waiting for this also.
Good answer anyway, Andrew. I may come back with more...when time permits.
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Andrew, after reading your post. The only other thing that springs to mind is the birefringent effect, i don't know enough about it to comment, but does it also play a part?
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There is always the fact that the polarizing glasses lens can be used as a refraction grating, separating light into the rainbow colors.
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how many andrews do we have on the forum now?
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how many andrews do we have on the forum now?
too many, but it's easy to remember their names!
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sure is. in fact, anyone whose name i do not know i will just call andrew!
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You beat us to it! Glad to see you're so keen, and here's the full question:
How come when you wear polarised lenses, can you see strange patterns of light in windows and shiny rainbows in metal?
Asked by Lauren Rosen, Australia
Read this Question from our Podcast (http://www.thenakedscientists.com/HTML/content/questions/question/1792/)
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John Parker, Dept Engineering & Materials at Sheffield University:
The question we’ve been asked concerns why you see strange patterns on glass when you look at it with polarised light (light that only vibrates at one angle - see this question) and the answer to that really comes in the fact that the glasses that have been looked at have been toughened by thermal quenching – by cooling very rapidly. The consequence of doing that is that you get stresses in the glass which have a different orientation according to where the cooling nozzles were in the original system used for quenching. So in some parts of the glass the stresses are running up and down, in some parts they’re running across and so on. The light, as it travels through the glass interacts with those tresses differently and in effect you end yup with two rays travelling through the glass in different polarisation senses, travelling at slightly different velocities. When they emerge, the recombine and they’re a bit out of step. What you’re eye is seeing is a colour associated with just how out of step they were. That’s the explanation for the effects you see with toughened glass.
With metals, the answers almost certainly to do with the fact that when you get a reflection from the metal surface you some get some polarisation. You may, possibly have, an oxide layer which is as a result of tarnishing and things. All of these effects influence polarised light as it comes though.