Polarised light

Polarization (also polarisation) is a parameter applying to waves that specifies the geometrical orientation of the oscillation. Electromagnetic waves such as light exhibit multiple polarizations, as do gravitational waves and sound waves in solids. On the other hand, sound waves in a gas or liquid only oscillate in the wave's direction of propagation, and the oscillation of ocean waves is always in the vertical direction. In these cases one doesn't normally speak of "polarization" since the oscillation's direction is not in question.

In an electromagnetic wave, both the electric field and magnetic field are oscillating but in different directions; by convention the "polarization" of light refers to the polarization of the electric field. Light usually propagates as a transverse wave—both the electric and magnetic fields are perpendicular to the wave's direction of travel. The oscillation of these fields may be in a single direction (linear polarization), or the field may rotate at the optical frequency (circular or elliptical polarization). In that case the direction of the fields' rotation, and thus the specified polarization, may be either clockwise or counter clockwise; this is referred to as the wave's chirality or handedness.

The most common optical materials (such as glass) are isotropic and simply preserve the polarization of a wave but do not differentiate between polarization states. However, there are important classes of lossless materials classified as birefringent or optically active in which this is not the case and a wave's polarization will generally be modified or will affect propagation through it. In linear dichroism and circular dichroism, attenuation in propagation is dependent on the wave's polarization. One familiar example is the polarizer, an optical filter that transmits only one polarization.

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