Fresnel's rhomb

noun1

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.

   • In Fresnel’s rhomb, whatever be the colour of the light, the retardation of the phase is exactly 90°, or the corresponding retardation in space is exactly ^λ⁄₄ whatever the value of λ may be. — 1831