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How does polarisation of EM wave work?

by serverxeon
Tags: polarisation, wave, work
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serverxeon
#1
Apr19-09, 11:45 PM
P: 98
Does it removes either the electric wave of magnetic wave component of the EM wave?

And if so, won't the wave exiting the polariser not a EM wave anymore? More like an E wave or M wave.

If the above argument is correct, won't the speed of light become[tex]\frac{1}{\sqrt{\epsilon_{0}}}[/tex] or [tex]\frac{1}{\sqrt{\mu_{0}}}[/tex]
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jtbell
#2
Apr19-09, 11:58 PM
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No, an EM wave always has both electric and magnetic fields. The direction of polariztion refers to the direction of the electric field, by convention. A linear vertically polarized wave has the electric field oscillating up and down, and the magnetic field oscillating left and right. A linear horizontally polarized wave has the electric field oscillating left and right, and the electric field oscillating up and down.
Born2bwire
#3
Apr20-09, 12:17 AM
Sci Advisor
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Boundary conditions. An EM wave can always be thought of as being the superposition of two polarizations that are normal to eachother. You can reorient the basis vectors of your polarizations to be parallel and perpendicular to the axis of polarization of your polarizer. When striking a perfect electrical conductor (PEC), the tangential E and normal H components are zero on the surface. These three vectors are all contained in one polarization (let's call it the transverse electric (TE) case). The other polarization, the transverse magnetic (TM) case, consists of the the normal E and tangential H fields. The TM fields do not need to be zero on a PEC surface.

A simple polarizer is a grid of wires with gaps between the wires smaller than the wavelength of desired operation. When the light passes through, the TE case (with respect to the wires) will see the metal surfaces along its vector and must be zero along the surface. Since the spacing is so small, this removes most of the TE energy from the wave after it has transmitted through. But the TM case is mostly unperturbed by the wires since they do not have to be zero along the surface. It should be noted though that both the TE and TM cases will completely reflect off of the PEC wires when striking them, it is only when they move along the surface will they behave differently.

Another way to think about it is to note that waves are cancelled out inside of a PEC because an incident wave creates currents on the surface of the PEC. The first order currents are due to the electric field in the EM wave and oscillate in the same plane as the electric field in the EM wave. These currents create secondary fields which cancel out the incident field inside of the PEC. With a polarizer, the TE case has an electric field oscillating along the wire. This means that the first order currents can be excited along the wire which results in the fields that are necessary to cancel out the TE fields on the opposite side of the polarizer. However, the TM case has the E field moving perpendicular to the wires. The electrons cannot oscillate against the grating due to the air gaps. So the TM case cannot excite the currents necessary to cancel itself out.


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