Extinction cross
The extinction cross is an optical phenomenon
Optical phenomenon
An optical phenomenon is any observable event that results from the interaction of light and matter. See also list of optical topics and optics. A mirage is an example of an optical phenomenon....

 that is seen when trying to extinguish a laser beam or non-planar white light using crossed polarizer
A polarizer is an optical filter that passes light of a specific polarization and blocks waves of other polarizations. It can convert a beam of light of undefined or mixed polarization into a beam with well-defined polarization. The common types of polarizers are linear polarizers and circular...

s. Ideally, crossed (90° rotated) polarizers block all light, since light which is polarized along the polarization axis of the first polarizer is perpendicular to the polarization axis of the second. When the beam is not perfectly collimated
Collimated light
Collimated light is light whose rays are parallel, and therefore will spread slowly as it propagates. The word is related to "collinear" and implies light that does not disperse with distance , or that will disperse minimally...

, however, a characteristic fringing pattern is produced.

Further reading

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