When a beam of light falls on a reflective surface, such as facet of a gemstone, part of the light enters the stone and part is reflected (Figure 3). Both portions behave according to definite laws. The governing the reflected portion states that the angle of incidence, or approach, and the angle of reflection are equal, and that the two beams are in the same plane. The angle of incidence is measured from the NORMAL. (Normal is a term used in geometry to mean perpendicular.) The angle of incidence is formed between the normal and the incident beam of light.
Although, as mentioned previously, light travels in air at a speed of 186,300 miles per second, its velocity is reduced materially when passing through denser substances such as gemstones. It is this reduction in the velocity of light in denser materials that causes the phenomenon known as REFRACTION. It is refraction that makes possible the action of lenses and most optical instruments and the brilliancy of diamond and other gemstones.
Briefly, refraction is a simple change in the direction of light that occurs at the contact surface between two different substances, such as between air and a gem or between air and water. Whether consciously or not, you have often actually "seen" refraction; e.g. when a straight stick through into water APPEARED to bend at the water's surface, or when an object inside a glass of water SEEMED to be in a different place than you knew it to be. If you have not noticed such examples of refraction, it may clarify the phenomenon if you experiment with a lead pencil and a tumbler of water. An experienced diamond man knows that imperfections in diamonds appear to be in other than their actual positions in the stone. One imperfection in a diamond will often be seen at the same time through two, three, or more facets and appear to be several imperfections, each in a slightly different position in the stone.
To illustrate the nature of refraction and why it occurs, we must consider light not only in terms of a single particle moving as a single wave (Figure 34), but in terms of an infinite number of these waves side by side that, when combined, form a beam of light having width. Its appearance in its simplest form would be similar to the long waves, swells or breakers on a body of water moving toward a shoreline. The long lines or breakers, are called WAVE FRONTS (A and B of Figure 5), and a beam always travels in a direction that is at right angles to its wave front of light.
Now let us assume that a series of light waves is traveling in a parallel line, so that together they create a wave front at RIGHT ANGLES to the direction of transmission (Figure 6). When the wave front strikes a parallel sided glass plate at right angles to the surface, it enters the glass and is immediately slowed down. Since the frequency, or rate of vibration, of the light waves does not change, the decreased velocity results only in shortening the wavelength of the beam, as depicted by placing the wave fronts closer together in the glass. When the beam leaves the glass, it immediately resumes its original wavelength; thus both velocity and direction are the same when leaving the glass as when entering.
But suppose that instead of entering the glass at right angles to the surface, the beam strikes the surface on an OBLIQUE angle (Figure 7). It can be seen that as the left portion of the wave front enters the glass, it is slowed down. However, since the entire wave front does not pass into the glass at the same time, that portion on the right, which is still in the air, continues at the same velocity. By the time the entire wave front is in the glass, its direction has changed. Since the direction of travel of the broad light beam is always at right angles to its wave fronts, the beam within the glass now begins to travel in a new direction. The amount of this bending from its path in air is proportional to the difference in the velocity of light in the two substances, air and glass. This bending of light is called refraction; it occurs when light passes obliquely from one medium into another of different optical density.
Obviously, just the opposite bending occurs as the light LEAVES the glass. When material through which light passes has parallel sides the direction of the light beam as it enters air will be parallel to its direction before entering the material. If these sides are not parallel when the beam returns to air, its new path will not be parallel to the original path. For example, Figure 8 shows that occurs when a beam of light passes through a prism; the amount of bending that occurs depends on the angle at which light enters the new medium from air and on the optical density of that material.
The optical density and thus the strength of refraction varies from species to species. Zircon, for example, reduces the velocity of light traveling through it much more than does opal. The velocity at which light passes through a given species is a constant for that species. Also, since the reduction in velocity of light as it enters a gem determines the degree of refraction, or bending, of the light, the angle of refraction for any given angle of incidence is also a constant for every gem species. The comparative ability of a gem to bend, or refract, light is called its REFRACTIVE INDEX (abbreviated R.I). This is expressed not in velocity, but as the ratio of the speed of light in air to its speed in a substance. For example, diamond has an R.I. of 2.42. This means that light travels in air at a velocity 2.42 times greater than its velocity in the diamond, the latter being approximately 77,000 miles per second. Because R.I. is a measure of optical density, the higher the R.I., the greater the degree of bending for each angle of incidence (except the perpendicular, of course).
Refractive index is measured by an instrument known as a REFRACTOMETER. The theory behind the operation of this instrument will be explained in a later assignment.