A polarizer (or polaroid) is a thin plastic sheet that produces a high degree of linear (or plane) polarization in light passing through it. This Demonstration shows a beam of unpolarized light reduced in intensity by 50% after passing through a polarizer oriented at an angle
. (Angles are measured counterclockwise from the horizontal, looking into the beam from the screen.) A second polarizer oriented at angle
will further reduce the intensity by a factor of
. In particular, if the two polarizers are parallel (
), there is no reduction in intensity. However, if the two polarizers are "crossed", with
, no light will pass through.
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
are transformed into a superposition of two linear polarizations parallel and perpendicular to a new angle