Page:Elementary Text-book of Physics (Anthony, 1897).djvu/485

§ 383] opposite direction. A plate of such thickness as to produce a retardation of one quarter of a wave length will give a circular vibration, and the beam issuing from the plate is then circularly polarized. Its peculiarity is that the two beams into which it is divided by a doubly refracting crystal are always of the same intensity, and no form of analyzer will distinguish it from ordinary light. Quarter wave plates are often made by splitting sheets of mica until the required thickness is obtained.

382. Circular Polarization by Reflection.—It has been stated that light refected from a transparent medium at a certain angle is polarized, and that an equal amount of polarized light exists in the refracted beam. Light totally reflected in the interior of a medium is also polarized, and here, there being no refracted beam, the two components exist in the reflected light, but so related in phase that the light is elliptically polarized. Presnel has devised an apparatus known as Fresnel's rhomb, by means of which circularly polarized light is obtained by two internal reflections of a beam of light previously polarized in a plane at an angle of 45° with the plane of incidence.

383. Effect of Plates Cut Perpendicularly to the Axis from a Uniaxial Crystal.—A crystal, such as Iceland spar, which has but one optic axis, is called a uniaxial crystal. Polarized light passing perpendicularly through a plate cut from such a crystal