Gemstones possess several physical and chemical qualities, however the most valuable quality of gems are in their optical properties. Principal optical properties of gemstones are: single and double refraction and polarization of light.
The term “refractive power” means an ability of any transparent surface (surface of a gemstone in our case) to altering the direction of a ray of light, directed at it. Thus, a brilliance, that we observe on the surface of any gemstone is nothing else than its refractive power.
All gemstones are separated according to their ability to refract a ray of light into two groups:
- The gems that possess single refraction
- The gems that possess double refraction
Each of us learned from the school physics course, that any ray of light, directed to any transparent surface will bend or refracted from its direction and proceeds to another course. This is an illustration of a single refraction.
If a ray is passing through a crystalline body, it is divided into two different rays, that are spread in several directions inside the crystal. One of them, so-called “ordinary ray” is going in the direction, which coincides with the law of “single refraction,” the second one so-called “extraordinary ray” follows a different law. Therefore, all the crystals, possessing of this quality are doubly refractive.
Polarization of Light
The term “Polarization of Light” has been introduced by Newton, in 1704. He concluded, that when a ray of light falls on a plate of transparent glass under an angle of 56 degrees, and after refraction falls towards to the second plate under the same angle, so, if two these plates are horizontal, the ray will reflect from second plate, maintaining the angle of inclination of 56 degrees; whereas, if the second plate will be turned in vertical position, the ray will no longer be reflected, and will disappear.
- Thus, the ray in that case is called “Polarized.”
- The angle of 56 degrees is called the “Polarizing Angle.” However, this angle is different for all bodies, thus, for instance, the polarizing angle of Diamond is 68 degrees.
By the way, this principle was used by such well known corporation as Polaroid. Exactly this physical feature is used in their lenses to protect your eyes against of so-called “horizontal component” of light, which is represented only an “optical noise” and “adverse solar flare,” whereas the “vertical component” of light represent to human eye an useful information such as the color and contrast, according to their researches.
Moreover, such gem as Tourmaline is primarily used to polarize light.
An effect of polarization is also used to determine such property as double refraction, described above.