It has been pointed out that light may be considered as a form of wave motion. When ordinary light travels in air or in space, the wave motion is not confined to a single plane; instead, there are many waves vibrating in all directions about the path of the beam. When this vibratory motion is limited to a single direction, or plane, the light beam is said to be POLARIZED.
Figure 1A shows the simple wave motion of light in a single plane; i.e., corresponding to the plane of the printed page. A "head-on" view of this beam is shown in Figure 1B; the arrows merely indicate the vertical directions of vibration. Since the beam is vibrating in only one plane, it is polarized. An ordinary beam of light actually consists of wave vibrating in all directions, as indicated in Figure 1C.
Light waves passing through solids have been shown to be slowed an amount that is in proportion to the refractive index of the material. Another means of distinguishing among solids is the effect-they have on the vibration directions of transmitted light. Some crystalline solids have the ability to polarize transmitted light into two directions of vibration at right angles to one another and to transmit the light in these planes at different speeds. This property is related to the crystal structure of the material. Such materials are said to be DOUBLY REFRACTIVE, since they confine a beam of light to two planes of vibration and refract that light vibrating in these planes at different angles. Materials that fail to polarize transmitted light are said to be SINGLY REFRACTIVE.
Figures 2 and 3 illustrate these two types of material. Figure 2 shows a beam of ordinary light at the left vibrating in all planes and passing through a singly refractive stone. Since the singly-refractive stone has no effect on it, the beam emerges at the right still vibrating in all directions.
Figure 3 shows a similar beam of light passing through a doubly refractive stone. Such stones permit light to vibrate in only two planes at right angles to one another; therefore, the beam emerges polarized.
Since light traveling in two planes does so at different speeds, if the beam enters the material at an OBLIQUE angle (Figure 4), it will be separated into two distinct rays, thus accounting for so-called double refraction. The amount of separation depends on the difference in speeds. This splitting or a beam is characteristic of any doubly-refractive material (any material of the tetragonal, hexagonal, orthorhombic, monoclinic and triclinic systems). Stones of the cubic system, as well as amorphous materials, constitute the singly refractive materials. The amount of splitting, or comparative strength of double refraction, of a gem is referred to as its BIREFRINGENCE (pronounced by-ree-FRINGE-ence). Numerically, birefringence is the difference between the refractive indices of the two rays. These values are listed in the accompanying table.
A simple illustration of double refraction is shown in Figure 5, using a piece of calcite looking through the calcite, one sees a double image.
Every doubly-refractive gem exhibits such doubling, although it is so weak in most stones that magnification is required for detection.