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.
Another possible design is the use of a "half-silvered mirror". This is a plate of glass with a thin coating of silver (usually deposited from silver vapour) with the thickness of the silver coated such that of light incident at a 45 degree angle, one half is transmitted and one half it reflected. Instead of a silver coating, a dielectric optical coating may be used instead.
A third version of the beam splitter is a dichroic mirrored prism assembly that splits 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.