GITAM, Department of Engineering Physics
Quarter-Wave Plate
A quarter-wave plate consists of a carefully adjusted thickness of a birefringent material such that the light associated with the larger index of refraction, μo, is retarded by 90° in phase (a quarter wavelength) with respect to that associated with the smaller index, μe. The material is cut so that the optic axis is parallel to the front and back plates of the plate. Any linearly polarized light which strikes the plate will be divided into two components with different indices of refraction. One of the useful applications of this device is to convert linearly polarized light to circularly polarized light and vice versa. This is done by adjusting the plane of the incident light so that it makes 45° angle with the optic axis. This gives equal amplitude o- and e-waves. When the o-wave is slower, as in calcite, the o-wave will fall behind by 90° in phase, producing circularly polarized light. We relate the thickness of the quarter wave plate, d, with the wavelength of the source, l, as,
(μo - μe)d = l/4
Half-Wave Plate
A half-wave plate introduces a
phase difference of
radians between perpendicular axes. Rotating a half wave plate by an angle
relative to the polarization direction of linearly polarized light will
therefore shift its polarization angle by
.
This can be observed by placing a half-wave plate at a 45° angle between
crossed polaroids, and noting that light is transmitted.
(μo - μe)d =
l/2