Fresnel's rhomb

plural Fresnel's rhombs

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, George Biddell Airy, Mathematical Tracts on the Lunar and Planetary Theories, the Figure of the Earth, Precession and Nutation, the Calculus of Variations, and the Undulatory Theory of Optics, 2nd edition, Cambridge: […] J. Smith, […] for J. & J. J. Deighton; and J. G. & F. Rivington, […] London, page 400