Chapter 55
Polarization
In normal white light, the plane of the electric field vector occurs in random directions for different wave
trains; such light is said to beunpolarized.Inpolarizedlight, the electric field vector for all waves is in the
same plane.
Light may be polarized by several different methods:
- Selective absorption
- Reflection
- Scattering
- Birefringence
55.1 Selective Absorption.
Inselective absorbtion, unpolarized light is passed though a material called apolarizer. The polarizing
material has polymers embedded in it that absorb light whose electric vector is parallel to the polymers. The
light that passes through the polarizer has its electric vector in one plane only: the plane perpendicular to the
polymer direction. The direction of the plane of polarization (the plane of the electric vector) is called the
axis of polarizationof the polarizer.
If light passing through a polarizer is passed through a second piece of polarizing material (sometimes
called ananalyzer), the amount of light leaving the analyzer depends on the angle between the polarization
axes of the polarizer and analyzer. If the polarization axes are in the same direction, all of the light leaving
the polarizer passes through the analyzer. If the polarization axes are at right angles, the analyzer blocks all
the light from the polarizer, and no light goes through. In general, if the polarizer and analyzer are at at angle
with respect to each other, the intensityIof light leaving the analyzer is given byMalus’s law:
IDI 0 cos^2 (55.1)whereI 0 is the intensity of light leaving the polarizer, before it goes through the analyzer.
The intensity ofunpolarizedlight is cut in half after passing through a single polarizer.
55.2 Reflection; Brewster’s Law
Light may also be (partially) polarized by reflection from a reflecting surface (a linoleum floor or a glass win-
dow, for example). In this case, light will be polarized in a direction perpendicular to the plane of incidence: