A beamsplitter 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 special coated and cemented together such that half of the light incident is reflected and the other half is transmitted. Another design is the use of a half mirror. This is a plate of glass with a thin coating such that, of light incident at a 45o angle, one half is transmitted and one half is reflected. Both of a metallic coating and dielectric coating may be used for this plate.