Polarisation is a property of waves that can oscillate with more than one orientation. Electromagnetic waves such as light exhibit polarization, as do some other types of wave, such as gravitational waves. Sound waves in a gas or liquid do not exhibit polarization, since the oscillation is always in the direction the wave travels. When unpolarised light passes through a polarising filter, about half of its energy will be absorbed. The filtering is the result of long polymers in the material, all aligned the same way. These polymer molecules absorb the component of the light with which they are aligned so the light that emerges from the other side will all be polarised in line with the filter. The chance that any single photon will passing through depends on how close its polarisation is to the angle of the polariser.

A remarkable effect occurs when a third polarizer is inserted between two crossed polarizers: up to one quarter of the light intensity can then be transmitted. This is a consequence of the quantum nature of light. Photons linearly polarized at angle Θ1 are transformed into a superposition of two linear polarizations parallel and perpendicular to a new angle Θ2 with probabilities cos221) and sin221), respectively.