Prisms on a surface splitting light

Glan-Foucault prism

Glan-Foucault prism

A polarizing prism formed from calcite that is like the Nicol prism but has the two parts (prisms) separated by a thin amount of air and cut to such angles that most of the extraordinary beam will be transmitted and the ordinary beam totally reflected. Since part of the extraordinary beam will be reflected, the Foucault prism is somewhat less efficient than the Nicol prsim.