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White Light Colours through Crossed Polarisers
Supporting pages are an introduction to birefringence and a statement of the problem (.doc file).
This applet was written to show how colours are created when white light passes through crossed polarisers with a birefringent plate in between. The birefringent plate has its optic axis perpendicular to its thickness.
Try altering the spectral content of the light. You can choose just 3 wavelengths for red, green and blue, setting the others to be zero. You can progressively reduce the amplitudes of colours towards the blue, representing a filament lamp source. However, in reality, your eye can partly adapt to changing white light sources, re-defining what it interprests as white. You can alter the birefringence and see what thicknesses are need to produce a given effect. You can see very simply how a plate that is quarter-wave or half-wave is only so at one particular wavelength and what colour it will let through if other wavelengths are present.
The applet and web pages were designed and coded by Gary Skinner, from an intital idea by Sylvanus P Thompson, as part of his undergraduate physics project.