light reflecting from a reflecting floor or swimming pool will be horizontally polarized, and light reflecting
from a window will be vertically polarized.
Reflected light will, in general, be onlypartiallypolarized. At one particular angle of incidence, though,
the reflected light will be not just partially polarized, it will becompletelypolarized. That incidence angle is
called thepolarization angle, and is given byBrewster’s law: Brewster’s law
tanpDn
nair(55.2)
Herepis the polarization angle,nis the index of refraction of the reflecting material, andnairD 1 is the
index of refraction of air.
Since light reflecting from a horizontal surface light a swimming pool will be at least partially horizontally
polarized, polarizing sunglasses are designed to have their polarization axis in theverticaldirection to block
the reflected light.
55.3 Scattering
Light may be polarized byscatteringof light. This may be seen by observing a clear blue sky through
polarizing sunglasses; by rotating the sunglasses you can see the sky getting brighter and darker, as the
sunglasses’s polarization direction changes with respect to the direction of polarized skylight.
55.4 Birefringence
Another method of polarization isbirefringence. This notably occurs in the mineral Iceland spar, which is a
transparent crystalline form ofcalcite, or calcium carbonate (CaCO 3 ). If Iceland spar is placed on top of a
page of printed text, you will see the image of the text is doubled (i.e. there will be two images of each letter).
Each image is polarized in a different direction, as you can verify by rotating a polarizing material in front of
the Iceland spar.
Figure 55.1: Birefringence in a sample of Iceland spar. (Credit: Jo Edkins, http://gwydir.demon.co.uk/jo/
minerals/index.htm)