Strange patterns with Sunglasses?

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.