Magenta is white minus green so if green was at right angles to the second polarisor and red was - 45 and blue was +45 (quite likely as blue is rotated more than red generally then the resulting colour is magenta, similarly with cyan and yellow. Chris. Question: What happens to me when I die? Response: What happens to a cat when it dies? Answer: You throw it away. Response and answer: You are a clever cat! :> -----Original Message----- :> From: owner-photoforum@xxxxxxxxxxxxxxxxxxxxxx [mailto:owner- :> photoforum@xxxxxxxxxxxxxxxxxxxxxx] On Behalf Of ADavidhazy :> Sent: 17 August 2005 21:06 :> To: List for Photo/Imaging Educators - Professionals - Students :> Cc: andpph@xxxxxxxxxxxxxxx :> Subject: RE: Question about polarization :> :> 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. :>
