Light Behavior
When white light strikes the surface of a mineral, it may be scattered, reflected, refracted, transmitted or absorbed. Light scattering and reflecting are surface phenomena, while refracting, transmitting and absorbing are internal affairs. With regard to surface phenomenon, if the surface is irregular, then the light is reflected and scattered in all directions. Although the scattering and reflecting of light is dependent upon the surface finish, another factor to consider is how well the gem material refracts light. For example, higher refraction results in greater scattering and reflection or a higher luster.
Scattering and reflecting have much to do with luster, brilliance and scintillation, but little to do with color. Minerals are colored when certain wavelengths of light are absorbed and the combination of the remaining wavelengths are transmitted to the eye. The wavelengths transmitted are perceived as color (e.g., if red and violet are absorbed, a combination of blue and green are transmitted and the gem will appear green). Wavelengths that are absorbed generate the absorption spectrum of a gem and can be quantitatively measured by a spectroscope. Best spectroscope results occur with strongly colored gems that are transparent, whereas opaque gem materials are nearly impossible to test (Schumann, 1997, p. 36).
Refraction and Internal Reflection
When a ray of light passes from air into a denser medium, such as a gemstone, part is reflected from the surface and part enters the gem material. Light entering the gem is slowed and bent, with the amount of bending dependent upon the angle with which it hit the surface and velocity of light in the two media.
Higher angles and greater velocity difference between air and the gem will result in greater refraction.
This refraction or bending of light can be measured and this number is termed the refractive index. This index is a constant in different types of gems and is used in identification. The refractive index is defined as a ratio of the speed of light in air to the speed of light in the stone. That is, if the speed of light in air is 300,000 km/sec and speed of light in diamond, 125,000 km/sec, then dividing 300,000 by 125,000 is 2.4 or the refractive index of diamond is 2.4. Light in air is 2.4 times faster than the speed of light in diamond.
When light leaves the gem it is also bent a certain number of degrees away from the normal or an imaginary line drawn perpendicular to the surface of the stone. As the angle of refraction increases, the light leaving the stone will eventually graze the surface and this is called the critical angle.
Beyond the critical angle, total internal reflection will occur. Thus, light travelling through a stone will eventually intersect with the stone-air surface; and light striking within the critical angle will exit the stone, while light striking outside the critical angle will experience total internal reflection.
Total reflection means light will be reflected back into the stone, bounce off facets on another surface and eventually exit. Thus the path of the light can be controlled by facet angles so as to bring the light up to top of the gemstone rather than allowing it to escape through the back side of a mounted gem.
Read Asia Gems, http://www.asia-gems.com/gemology/light.php and http://www.asia-gems.com/gemology/light-behavior.php, for more information and diagrams.
All transparent substances can be classified as either isotropic or anisotropic. Isotropic includes amorphous mineraloids and minerals in the isometric crystal system. Light entering isotropic gems moves in all directions with equal velocity, creating only one index of refraction. That is, a single refractive index results when light is considered moving in a wave motion with vibrations in all directions at right angles to the direction of propagation. Put another way, imagine snapping a rope and seeing a wave traveling from one end to the other, with motion perpendicular to the rope (e.g., side to side or up and down). The wave travels in a single direction but is free to vibrate in random directions perpendicular to this single direction of travel. Hence, single refraction is the optical characteristic of light passing through a denser medium without polarization.
In contrast, when light enters anisotropic gem materials the light is split into two polarizing rays, vibrating in mutually perpendicular planes. Thus in a given orientation, two refractive indices, one associated with each polarized ray, is detected and the specimen is termed doubly refractive. Double refraction occurs in specimens from five of the six crystal systems, including tetragonal, orthorhombic, hexagonal, monoclinic, and triclinic. These anisotropic minerals possess the power to polarize light or confine the light wave to vibrate in only one direction, blocking all other waves and spliting light into two rays that travel at different speeds at right angles to one another.
Refractive indices vary from 1.2 to 2.6 and can be measured with several instruments. Both single and double refraction will be detailed in an upcoming lesson on the instruments used to test gems, such as the polariscope. This instrument detects single or double refraction, while gem refractive indices can be specifically measured with a refractometer. This instrument actually determines the critical angle and projects the refractive index onto a scale. Find out more about refraction and reflection at Atmospheric Optics, an interesting external link http://cimss.ssec.wisc.edu/wxwise/class/optics.html (a page from an Atmospheric and Oceanic Science at University of Wisconsin, Madison), which may help to put reflection and refraction into an everyday perspective using weather.
Dispersion
The final optical property to introduce is dispersion. Light is slowed and refracted or bent upon entering a denser medium. A characteristic refraction or bending is associated with each different wavelength of light and this separation of white light into component colors is called dispersion.
White light is in fact a combination of red, blue, and green wavelengths of light.
Recognition of this was credited to Sir Isaac Newton in 1666 when he observed a dispersed spectrum through a glass prism. Although he recognized the colors as a continuum, he assigned seven names, violet, indigo, blue, green, yellow, orange, and red by analogy with seven notes in a musical scale. Newton's observation of dispersion or dispersive refraction demonstrated that a light beam bends or refracts as it passes from one medium to another, such as from glass to air or from gem materials to air.
So, while dispersion through water drops in atmosphere produces rainbows, the same rainbow effect occurs in diamond. In addition, the wavelength of light will determine the amount of refraction or bending; shorter wavelengths in the blue end of the spectrum bend more than longer wavelengths in the red end.
Dispersion is actually measured by subtracting the refractive index of red light from the refractive index of violet light. Diamond's dispersions is 0.044 while quartz is 0.013; diamond is more dispersive than quartz.
Implications of Optical Properties for Gems
As stated above, the optical characteristics are enhanced when humans fashion the rough gem material and maximize the optical properties. If the gem is cut too shallow, light will leak out the bottom of the stone.
This is termed unplanned leakage. If the gem is cut too deep, and light passes inside the critical angle, it will leak out the bottom just the same and the results of a stone that is cut correctly where the light comes back through the top or crown portion of the gem.
Learn more at the required reading sites listed below.
Required reading for further clarification:
Lewton-Brain, C. (1986-1994). Basic optics notes for gemology. Ganoksin Jewelry Co., Ltd.: WWW URL http://www.ganoksin.com/borisat/nenam/optic.htm. Retrieved 2 February, 2004.
Helper, M. (2009) The behavior of light in minerals and gems: Refraction, reflection and the critical angle. UT Austin, Geo347k: Gems & Gem Minerals: WWW URL http://www.geo.utexas.edu/courses/347k/redesign/PDF_Handouts/347refrt.pdf. Retrieved 16 February, 2009.
Friedman, H. (1997-1999). Mineral properties - Optical properties. The Mineral and Gemstone Kingdom: WWW URL http://www.minerals.net/resource/property/optical.htm. Retrieved 16 February 2009.
Keller, B. (no date). Refractive index and critical angle. WWW URL: http://www.rockhounds.com/rockshop/gem_designs/refractive_index/.
The material for this section came primarily from:
- Hurlbut, C. S., & Kammerling, R. C. (1991). Gemology. NY: John Wiley & Sons, Inc.
- Schumann, W. (1997). Gemstones of the world. NY: Sterling Publishing.
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This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: November 15, 2000; last update: February 16, 2009.
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