Why is it that when we change the light from un polarized to polarized

[elabed haidar]

why is it that when we change the light from un polarized to polarized light the intensity goes to half its value and is the polarization only have to the electric field and why?? thank you

 

[sophiecentaur]

To avoid confusion (!?) we always define polarisation in terms of the Electric vector. There is, of course, a Magnetic field vector, too.
A Polariser selects the Component of the field vector in a particular plane. Vectors of all sorts can be resolved in the same way; think of an inclined plane which can give you a whole range of effective g values by varying the angle by 'choosing' the appropriate component.

Light from most sources behaves as many sources, each with a random angle of polarisation. What gets through a polariser will be the sum of all components of all the waves in the chosen plane. The average value will be 1/root2 so the power will be 1/2 of the original power.

 

[elabed haidar]

so that's why i heard because we can measure the electric component but i didnt get why can't we measure the magnetic component

 

[sophiecentaur]

You can't actually measure either, easily with light but you can make an E probe or an M probe to measure RF fields.

 

[chrisbaird]

Think of unpolarized light as the incoherent sum of two polarized beams, with orthogonal polarizations. When you polarize the light, you are letting through the beams with the one polarization and not the other, so you have half the energy getting through. The polarization of an electromagnetic wave is the electric field vector after the wave dependence has been factored out, different from the induced polarization of a material. The magnetic field vector is always there, but in free space it is trivially related to the electric field vector, so we ignore it.

 

[fluidistic]

See https://www.physicsforums.com/showthread.php?t=435547&highlight=fluidistic+polarization.
If my memory is good, I've read in Hecht's book on Optics that we generally consider only the E fields rather than the B field because the magnitude of E is c times greater than the magnitude of B in many cases. I might be wrong on that and if I am, please tell me.

 

[elabed haidar]

thank you fluidistic very much i got it