# Why doesn't the magnetic field polarize when polarizing light?

If the magnetic field doesn't polarize does it follow the electric field path of propagation? or does it vanish?

The magnetic field polarizes orthogonal to the electric field in free space. We generally only talk about the electric field because Maxwell's equations define a one to one relationship between the two. It would make just as much sense to only talk about the magnetic field. We choose the electric field because, in general, when light interacts with matter it is the electric field which causes all of the interesting effects (though this in not strictly true).

• Among the examples for "not strictly true" might be a set of experiments commonly known as electron/nuclear paramagnetic resonance where the magnetic field couples to the spin of interest and causes the "interesting" evolution.
– eqb
Commented Jun 8, 2014 at 22:40

The magnetic field does not vanish when light is polarized. A changing electric field induces a magnetic field, and a changing magnetic field induces an electric field. This is why, in the propagation of an electromagnetic wave, there is always an oscillating electric field coupled with a magnetic field oscillating perpendicular to this electric field. You cannot simply take away one of these fields.

Unpolarized light consists of many electromagnetic waves polarized in different directions. Each of these waves has its own electric and magnetic fields, which are perpendicular to each other. When this light is polarized by, say, sending it through a polarizing filter, the electromagnetic wave that results is polarized only in one direction. There still exists both an electric and magnetic field to this wave.

The graphic below illustrates this effect, but only shows the electric field. In reality, there is still a magnetic field that oscillates perpendicularly to the resulting, polarized electric field.

• The polarizer act on the EM waves. For about 50% of the incoming waves (which are equally distributed in 360°) the polarizer rotate the EM waves parallel to the slits. You can convince yourself by placing a third polarizer under 45° between the two existing polarizers. Without this polarizer light doesn't goes through the crossbred polarizers. With the polarizer under 45° the slits act on the EM waves and you see light behind the last polarizer. This amazing fact shows the slits influence of EM waves. Commented Jul 12, 2014 at 17:10
• @HolgerFiedler: (I know this is really late, sorry) Does this mean in regular EM waves the magnetic and electric waves arent orthodiagonal? Is it only when it becomes linearly polarized it does? Commented Nov 1, 2019 at 3:17
• @AopsVol.2 Its not clear to me, how this question arises to you. May you explain? The E and the B field of EM radiation are always orthogonal in vacuum. Behind a polarizer they are also orthogonal. Commented Nov 1, 2019 at 6:31
• @HolgerFiedler: A definition in a physics book I'm reading says that when the vibrations are in one direction and are perpendicular to the direction of the waves propagation the then wave is linearly polarized. If polarized light has both an E and B field it no longer vibrates in one direction right? One will vibrate orthogonal to another (maybe this question arises from my lack of knowledge on vectors) and thus in separate directions... Commented Nov 1, 2019 at 12:35
• ...I tried to get around this by thinking of the B and E waves as separate then polarizing them thusly but I'm not sure you can do that. (I'm not a physics student, just a 15 year old reading "for the love of physics". That's probably why I'm asking these dumb question) Commented Nov 1, 2019 at 12:35