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I was wearing polaroid sunglasses and happened to look at the lenses on a pair of reading glasses and noticed a rainbow pattern. I have also noticed vertical bars when looking out car windows while wearing the sunglasses. What is happening here? Also as a related question, when my LCD tv is off, it reflects the light from a nearby lamp and I can see a similar rainbow light diffraction... Is this because the liquid crystals are acting as small prisms?

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the polaroid lenses reveal colored patterns in things like automotive safety glass and other eyeglasses when those things contain either residual stresses which were frozen into them during their manufacture or when the manufacturing processes produced some net alignment or anisotropy of the molecules in them (in the absence of residual stresses). either of these things can slightly alter the polarization of light passing through them, which the polarizing filter then makes visible for you.

In the days before cheap computer modeling, stress concentrations in engineering parts could be visualized for purposes of analysis by machining a replica of the part out of a chunk of clear plastic that tweaked the polarization of light when under stress. You then applied the stress and took pictures through a polarizing filter of the resulting colored fringe patterns in the part.

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Try moving your head sideways to see if the rainbow pattern moves. If the rainbow pattern then maintains a fixed position on the surface, it is due to stresses in that surface. However, if it moves along the surface it is due to a birefringent film on the surface. Car windows may have been tinted with a polyester fiim that is birefringent. I guess a tv screen is more likely to contain a birefringent layer than to contain residual stresses.

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