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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#msg129590Since 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!
Modern cars use mini arc lights
Mine don't dazzle, the have a very well defined beam and servos to compensate for the vertical movement of the car