Index Key:  PHY070
Author:     trumpetman
Subject:    Polaroid Glasses and TI Calculators
Text:       When looking at a TI graphing calculator with an LCD display while 
wearing 3D or Polaroid sunglasses, quite interesting colors result depending 
on the orientation of the filters in the glasses.  Why does this occur?

Response #:  1 of 1
Author:      Mooney
Text:        The colors result from a correlation between the wavelength of 
light that makes it through the LCD and the polarization of that light.  I do 
not know the details of an LCD, but I can describe a similar, and simpler, 
effect that you might be interested in.  Take two Polaroid filters (Polaroid 
sunglasses are fine) and place a crinkled up piece of cellophane between them.  
You will see beautiful colors that vary with the orientation of the filters 
and with the thickness (number of layers) of cellophane between them.  This 
effect is easier to discuss because we are in complete control of the 
polarization of incoming light.  We start with a definite polarization of the 
light that makes it through the first Polaroid--the polarization is 
independent of wavelength.  Then, the cellophane *rotates* the plane of 
polarization of light passing through it; the amount by which the polarization 
is rotated depends on the wavelength.  These two things happen because the 
index of refraction of cellophane depends both on the wavelength and on the 
polarization of the light passing through it.  The second Polaroid accepts 
only light with a particular polarization, so, if "yellow" got its 
polarization rotated by the right amount to make it through both Polaroids, 
"blue" will probably not have.  If you want to know more, you might look for 
information on the term "birefringence".  Although I know little about LCD's, 
I strongly suspect a very similar thing is going on there.