## What is polarization?

We often draw just the electric field of light waves or electromagnetic waves as physical transverse waves.   The orientation of this electric field from a normal light source is usually in a random direction, and unpolarized light will contain equal contributions of all orientations.  One could think of sending waves down a slinky or a rope.  One could shake the slinky horizontally, vertically, or at any infinite number of angles in-between.   If the rope were to pass through a set of prison bars, then the vertically oscillating waves would pass through, but the horizontal waves would be blocked.

## Polarization by passing through a material

Although the rope and bars is a great model of how polarized waves are generated, it breaks down at the microscopic level.  Diffraction gratings due consists of hundreds of small “bars.”  A piece of Polaroid film actually consists of polymers that have been stretched in such a way that they interact and absorb waves with the electric field orientated in a particular direction.  This happens in a dichroic material.

## Birefringence

Some crystals have a natural birefringent property because of the asymmetry in the crystals.   Consequently, the index of refraction of vertically polarized light will be different from the index of refraction for horizontally polarized light.  Using the bending due to refraction, this can actually be used to make a very efficient beam splitter.  Calcite, or Icelandic Spar is commonly used.  When you look through a chunk of calcite, you will actually see two images, one for each polarization.  As you rotate a polarizer over the calcite, the two images fade in and out.

## Polarization by reflection

At very shallow angles, reflected light will be polarized.   This angle is called Brewster’s angle. So light that reflects from a surface at a very oblique angle (such as glare from the water, a road, or the hood of your car) will be polarized perpendicular to the surface.  So although polarized sunglasses will absorb about 50% of the unpolarized light that generally comes into them via polarization,  the purpose of polarized sunglasses is primarily to absorb 100% of the glare.

## Kerr and Pockel Cells

Another interesting use is as a shutter.  Certain materials become birefringent when an electric field or voltage is applied to them, as in the Kerr Cell or Pockel Cell.   This can be used as an optical shutter in cameras and laser systems.  I spent a couple weeks during the summer of ’92 testing a Kerr Cell for the Nova Upgrade at LLNL. This was used to create a pulsed laser with Petawatt pulses.   This became the National Ignition Facility for generating Nuclear Fusion.

## Crossed Polarizers

One linear polarizer will block 50% of the light.  When two polarizers are perpendicular to each other, no light will pass.  If they are at an angle to each other, the amount of light that gets through can be described by the Law of Malus. The sine component (or perpendicular component) of the electric field of the wave is absorbed, allowed the parallel or cosine component to pass.   Thus the passing electric field is

As optical intensity is proportional to the square of the electric field, the intensity of the beam which passes can be described by the Law of Malus.