Fresnel's rhomb

Etymology
Designed in 1817 by Augustin-Jean Fresnel.

Noun

 * 1) A rhombus or oblique parallelopiped of glass cut so that a ray of light entering one of its faces at right angles shall emerge at right angles at the opposite face, after undergoing within the rhombus, at other faces, two reflections. It is used to produce a ray circularly polarized from a plane-polarized ray, or the reverse.