Beam splitter
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A beam splitter is an optical device that splits a beam of light in two. It is the crucial part of most interferometers.
In its most common form, it is a cube, made from two triangular glass prisms which are glued together at their base using Canada balsam. The thickness of the resin layer is adjusted such that (for a certain wavelength) half of the light incident through one "port" (i.e. face of the cube) is reflected and the other half is transmitted. Polarizing beam splitters, such as the Wollaston prism, use birefringent materials, splitting light into beams of differing polarization.
Another design is the use of a half-silvered mirror. This is a plate of glass with a thin coating of aluminum (usually deposited from aluminum vapour) with the thickness of the aluminum coating such that, of light incident at a 45 degree angle, one half is transmitted and one half is reflected. Instead of a metallic coating, a dielectric optical coating may be used. Similarly, a very thin pellicle film may also be used as a beam splitter.
A third version of the beam splitter is a dichroic mirrored prism assembly which uses dichroic optical coatings to split the incoming light into three beams, one each of red, green and blue. Such a device was used in multi-tube colour television cameras and also in the 3 film Technicolor movie cameras.
In Physics, the beam splitter is famous for its use in the Michelson-Morley experiment in 1887.
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