Rhomb
or RHOMBUS, in geometry, an oblique parallelogram whose sides are all equal. The diagonals of a rhombus bisect each other at right angles. The area of a rhombus is equal to half the product of its diagonals.
Fresnel's rhomb, in optics, is an apparatus for converting plane into circularly polarized light. It is a parallelopiped of glass, of such length and angles that a ray of light entering one small end at right angles, twice suffers total reflection within the rhomb at an angle of about 54° (depending on the polarizing angle of the glass), and finally emerges at right angles from the opposite small end. When the beam of light is plane polarized, and the rhomb is so arranged that its reflecting faces are inclined at an angle of 45° to the plane of polarization, the beam emerges circularly polarized.