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Q & A: Polarization by Reflection

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Most recent answer: 10/22/2007
Q:
how can we explain polarization by reflection using electromagnetic theory?
- kamal
india
A:
This one is a bit tricky. The best way to truly understand how to do this involves solving Maxwell's equations for light hitting an interface. But that is a bit tedious, so I'll give you the end result.

First, you need to know that light has both and electric field and a magnetic field and that these fields are perpendicular to each other and also perpendicular to the direction the light is traveling. Next, you can think of light as having a mix of 2 polarizations with respect to an interface. One is where the electric field is parallel to the interface (called TE or Transverse Electric) and one where the magnetic field is parallel to the interface (called TM or Transverse Magnetic). Any beam of light can be broken into these 2 parts. (If the beam is traveling directly perpendicular to the interface, BOTH fields are always parallel to the interface, so think of some case where the light comes in at a slant.)

Now, if you solve Maxwell’s equations for each part and look at the reflection, you don’t get anything very interesting for the TE case. But, there is one angle of incidence for the TM case for which there is no reflection. This is called the Brewster Angle.

One place where this idea is used all the time is in lasers. Lasers need a cavity where the field can build up very large. Many lasers have a piece of glass that is exactly at its Brewster Angle. When light passes through it, some of the TE light reflects out of the cavity. The end result is that the TM light builds up very large and the TE doesn’t. So the beam that comes out of these lasers is polarized because of that piece of glass at the proper angle.

Adam (w Mike)

(published on 10/22/2007)

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