Chris, I think we are on similar wavelengths ... the color that we see on the output side is that which is aligned with the plane of polarization of the second polarizer (and modified by residual transmission of other wavelengths) - so seeing green means that green is passed to a great extent while other wavelengths are (if at all) passed to a lesser extent. But what about magenta? Since it does not exist in the spectrum it must mean that two primary colors are mostly aligned with the analyzer, no? andy -------------------------- in reply to: ---------------------------- Nah! Nah! The first polarisor gives plane polarised light that is white (all colours) the cellophane or other plastic rotates the light an amount depending on the stress and the wavelength of the light, so when you put the second polarisor in so the polarised plane is at right angles to the first, the only light allowed to pass has been turned through an angle. Since this angle depends on wavelength and stress the colour depends on the degree of stress. The colours are attenuated depending on the angle through which the light of that colour is rotated. So the colour you see is the result of the sum of what light is passed. Since we are dealing with eyes then the primaries of red green and blue are the ones involved. So if red is twisted 15 degrees, green 30 degrees and blue 90 degrees then there will be a resultant colour attenuation of cos(15)=.966 for red + cos(30)=0.866 for green and + cos(90)= 0 for blue. I think that makes it RGB: 1, 34, 255, ( ) this is obtained by transmittance = 1-attentuation and multiplying transmittance by 255. Other colours can be obtained by different degrees of stress. This property is under the heading of birefringence with an ordinary and extraordinary ray, but I cannot remember the connexion. To calculate the transmittance directly, use the sine of the angle of rotation. If you want to measure the stress use monochromatic light, obtained from white light passed through a diachronic filter to give (say green) then measure the angle of rotation by rotating the second polar to make the light extinct, this is then at right angles to the plane of polarisation of the light. NB added afterwards the system is calibrated by putting a known stress and measuring the rotation for several values and plotting a graph. Chris.
