Page:Elementary Text-book of Physics (Anthony, 1897).djvu/476

463 it and the optic axis is iu the direction of the greatest elasticity to which any vibration giving rise to that ray of light can correspond. In that direction is the second component vibration. The two component vibrations are therefore always at right angles. One of the components is always at right angles to the optic axis, and hence in the direction of least elasticity. The light resulting from this component always travels with the same velocity whatever its direction, and hence suffers refraction on entering the crystal or emerging from it, according to the ordinary law for single refraction. The other component, being in the plane containing the ray and the optic axis and at right angles to the ray, may make all angles with the optic axis from 0°, when it is in the direction of maximum elasticity and is propagated with the greatest velocity, to 90°, when it is in a direction in which the elasticity is the same as that for the other component, and the entire beam is propagated as ordinary light. Light for which vibrations occur in all azimuths will, on entering the crystal, give rise to equal components, but light already polarized will give rise to components the intensities of which are determined by the law for the resolution of motions. When its own direction of vibration coincides with that of either of the components, the other component will be zero, and only when its vibrations make an angle of 45° with the components can these components be equal. The varying intensities of the two beams into which a polarized beam is divided by a second crystal are thus explained.

377. Polarization by Reflection.—Light reflected from a transparent medium is found in general to be partially polarized, and for a certain angle of incidence the polarization is nearly perfect. This angle is that for which the reflected and refracted rays are at right angles. In Fig. 142 let $$xy$$ represent the surface of a transparent medium, $$ab$$ the incident, $$bc$$ the reflected, and $$bd$$ the refracted ray. If the angle