The Optical Properties of Al though the electromagnetic theory of light is now generally accepted, the terminology of the older undulatory theory is still conveniently used in discussing the optical characters of crystals. It is assumed that while the direction of advance in a homogeneous medium is in straight lines, the transmission is due to vibra tion (or waxing and waning of force) at right angles to this direction of advance. This may be called a vibration and designated in direction and intensity by a straight line. The advance during a complete vibration is called a wave length and determines the color of the light while the amplitude of the vibration deter mines the intensity.
The properties of light are elsewhere de scribed (see LIGHT). Those most used in the study of crystals are refraction, double refrac tion, polarization and absorption. In other words, because crystals on account of their structure retard the transmission of light dif ferently in different directions, both with re spect to the velocity of transmission and the quantity transmitted, these properties of trans mission are utilized to determine crystalline structure.
Rays of light in passing obliquely from one medium to another in which the velocity of transmission is different are bent or refracted. If the velocity is lessened the bending is toward the perpendicular to the sur face of contact, if the velocity is increased the bending is away from the perpendicular.
The ratio of the velocities of light in the two media is called the index of refraction of the second medium with respect to the first and it has been proved that whatever the angle of incidence, the ratio of the sines of the angles of incidence and the angle of refraction is con stant for the same two media and equal to this sin index of refraction. That is, sin r = VI Double Refraction.—In 1679 Erasmus Bar tholin observed that the objects viewed through iceland spar appeared double, one image con forming to ordinary refraction, the other not so conforming. For instance, one of the two rays had a fixed index of refraotion (1.658 for yellow light) while the index of the other varied for different directions of transmission from 1.658 to 1.486. As early as 1678 Huygheus showed that the optical behavior could be repre sented by a double shelled surface like that later described under optically uniaxial crystals.
Double refraction could be proved for only a few crystalline substances by the direct ob servation of double images. Other methods,
however, prove that it is a general property of all crystals except those of the isometric system.
Polarized In ordinary light the vi brations are assumed to take place at right angles to the hne of transmission but in any plane. Mains discovered that light reflected from inclined plates of glass, when viewed through the doubly refracting iceland spar, gave images which differed in intensity, and that for certain positions of the iceland spar. one or the other image disappeared. Light so modified is said to be polarized and the vibrations are con sidered to take place in one plane only.
The rays transmitted by any doubly refract ing crystal consist of polarized light and for any direction of transmission the planes of vi bration of the two emerging rays are at right angles to each other.
Absorption and passing through any substance is partially absorbed, the amount increasing with the thickness. If the light is composite the component lights of different wave length may not be absorbed in the same ratio and the substance will appear of the color due to the unabsorbed portions of the components.
In doubly refracting crystals the two rays transmitted in any direction are apt to undergo different absorptions and may emerge of dif ferent color and intensity. Furthermore in different directions in the same crystals the so called 'Itwin colorso may notably differ and in such a case the apleochroism° is observable by the eye alone. Thus a crystal of iolite appears bluish in one direction and yellowish in another.
The Optical Classification of Crystals.— According to their optical behavior crystals may be classified as 1. Optically isotropic, 2. Op tically anisotropic uniaxial, 3. Optically aniso tropic biaxial.
Optically Isotropic Crystals.— Isometric cry stals show the same optical properties in all directions, yielding in all directions the same index of refraction and the same rate of ab sorption. They are singly refracting or rarely circularly polarizing.
Optically Anisotropic Uniaxial Crystals.— In tetragonal and hexagonal crystals the op tical behavior varies with the direction, the principal crystallographic axis is also an axis of optical symmetry, for in all directions equally inclined to it the same optical tests are ob tained. It is a direction of single refraction or of circular polarization.