Polarization
Light can be circularly polarized, plane polarized or unpolarized. A plane-polarized wave has an electric field whose direction is fixed in a plane as in the previous animation. A circularly polarized wave is a superposition of an x-polarized and a y-polarized wave, where the relative phase is 90 degrees. For a circularly polarized wave moving along the z axis, the x component of the electric field wanes while the y component builds which results in the electric field's direction rotating about the z axis. By having the relative phase as +90 degress or -90 degrees, the rotation is either right-handed or left-handed. Unpolarized light is a random mixture of x- and y-polarized waves with random relative phases.
Certain materials polarize light along a certain axis due to their crystal structure. Once light passes through a polarizer, the direction of the light's polarization is known. If the incident wave had a polarization angle finc relative to a fixed axis and the polarizer is set at an angle fpol, the resultant electric field is reduced in magnitude by a factor of the cosine of the relative angle. The intensity is therefore reduced by the square of the relative angle's cosine.
For unpolarized light, the final intensity will be half the original.
Materials whose molecules are helical can produce circularly polarized light. Examples are sugar syrups.
Light from blue sky is polarized, as can be understood by thinking of it as sunlight that has been re-radiated (scattered) from charged particles in the atmosphere. "Polaroid" sun glasses and polaroid filters for cameras therefore have a striking influence on the apparent brightness of blue sky.
Light that is reflected from an insulating surface such as water or glass is polarized (more or less, depending on the angles involved), so polaroid sun glasses or camera filters have a strong and practical effect on the brightness of the reflections.