Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: chris on 05/12/2008 08:42:03

Title: Why do polaroid lenses make dark patches appear in the car windscreen?
Post by: chris on 05/12/2008 08:42:03
When wearing polaroid sunglasses in a car, sometimes dark patches appear in the windscreen glass. What are they and what is the reason for this effect?

Similarly, why do polaroids sometimes also reveal previously-invisible fish beneath the surface of the water?

Chris
Title: Why do polaroid lenses make dark patches appear in the car windscreen?
Post by: RD on 06/12/2008 16:10:23
The dark or multi-coloured areas on a stressed windscreen are a photoelastic stress pattern (http://en.wikipedia.org/wiki/Photoelasticity).
The windscreen is stressed so when broken it will shatter into pea-sized safe-ish fragments rather than lethal knife-sized shards.

The removal of reflection from water is due to "cross-polarisation". Sunlight on a sunny day (no cloud) is polarised.
Polarised light reflecting from a surface which is smooth compared with the wavelength of light, e.g. water, will retain its polarisation.
Polarised light reflecting from a non-smooth surface, e.g. rocks & fish in the river , will lose its polarisation.

A polarising filter whose axis of polarisation is crossed at 90o to the axis of polarisation incident polarised light will totally block that light.
So Polaroid sunglasses can totally block the polarised reflection from the surface of water but can only partially block the unpolarised reflection from the rocks & fish.