Why is reflected light polarised?
I have learnt about Brewster's angle, and how at a particular angle all light reflected is polarised, but do not understand why. Is this something that could be explained to a guy that doesn't have a Ph.D in physics?
Best Answer
The following diagram may be helpful:
If you have an incident ray that is polarized with the E field up and down (in the plane containing the incident ray and the normal to the surface), then when that ray is refracted, it contains a component of electric field that is perpendicular to the refracted ray (and still in the same plane).
The reflection is actually caused by the motion of electrons in the medium. Now you can see that the electrons, which move along the direction of the E vector, are moving parallel to the direction of the reflected beam. An electrical dipole that is oscillating can send a wave in all directions - except the direction it is pointing (if you think about it, looking at the dipole "end-on" you don't see it moving: if it doesn't seem to be moving, it shouldn't be seen to be radiating towards you...). This entire analysis is only for the E field with the polarization drawn: if the incident light contains a component of E field perpendicular to the sketch (in and out of the page), that component will be able to radiate along the reflected direction from inside the dielectric.
So there it is - for this very special orientation, the reflected light must be fully polarized. If you do the math carefully (see Fresnel's equations), you will see that you will have some polarization at any angle - again, because the amount of polarization in the reflected light depends on the ability of the dipoles in the material to excite a reflected wave. The Brewster angle is a special case that gives “pure” polarization; at other angles, there will be partial polarization. Incidentally this is why polarized sunglasses help reduce “glare” - if the majority of surfaces you look at are near-horizontal (for example, a body of water), then the polarization of the reflected light will be mostly horizontal; and since polarizing sunglasses only allow vertically polarized light through, reflections are reduced.