PHY 212 Net Research Project for 3/22/04

REFRACTION

Diamonds

There are few things harder than a diamond. Industrial diamonds typically have faults that make them somewhat unsightly, but do not seriously effect their hardness. They are commonly used in abrasion and cutting tools and they are suprisingly cheap. However, diamonds used for jewelry, are prized for their purity and flawlessness and will cost considerably more. But you can achieve a much higher level of purity and flawness with commercially produced and expensive glass. So why would one pay big bucks for a diamond? It's the index of refraction!

The index of refraction of a diamond is whopping 2.4 (compared to about 1.5 for glass). That means light is highly refracted when it passes through one of the diamonds facets. A good jeweler can easily tell the difference between diamond and glass by examining both under a bright light. When white light passes through a transparent substance, its color constituents are refracted by differing degrees due to the slight frequency dependence of the index of refraction. (This is the "prism effect" we discussed in class.) The diamond's higher index of refraction will exaggerate this spreading of colors in two ways. First, there is a greater angular spread in the refractive angles of the different colors upon entering and exiting the diamond than in the glass. Second, the larger index of refraction also means that the light will be totally internally reflected a greater number of times before emerging from one of the facets. Hence, the light travels further before emerging, which further increases the color dispersion. A diamond will sparkle with a greater variety of colors than a piece of glass with the exact same shape. If you or a friend have a diamond and a similarly shaped piece of glass, hold both up to a bright incadescent light source and you'll see the difference. A magnifying glass will help.

A diamond is mostly carbon. A pure white diamond would have nothing but carbon. But all diamonds have some level of impurities. Some impurities degrade the diamond and adversely effect their price, while some rare impurities can actually increase the value. (The picture to the right shows several color varieties and is courtesy of ProfessionalJeweler.com.) The structure is hexagonal in nature and the arrangement is quite specific. Graphite, the main ingredient in pencil "lead", is also hexagonal, but arranged in 2-D layers. Check out Namiki's molecular diagrams to see just how the carbon atoms are arranged.

So, why do carbon atoms arrange themselves into coal and graphite most easily but only rarely into a diamond? The main ingredient for forcing carbon atoms into the diamond structure is pressure. Diamonds are formed when carbon is subject to the great pressures found at around 100 miles beneath the earth's surface. Time is also an important element. Most diamonds are at least one billion years old. We obtain the diamonds when they are pushed to the surface through vulcanic or plate uplift. You can find more information about diamond creation from the Department of Geology and Geophysics at the University of Wisconsin.

High Index at a Cheaper Price

You can't afford a diamond, so is there something else with such an outrageously high index of refraction? YES, the amazing Cubic Zirconium! And how much would you pay? Don't answer yet, you also get this cap snaffler absolutely free .... (humor only for those who watch late night TV). Cubic zirconium is zirconia (Zr oxide) with some stabilizing additives, for example CaO-ZrO₂. The color can be controlled by the exact additive. The general class of these compounds is called stabilized zirconia. They have many industrial uses from sensors and engine parts to grinding powders. You can find more than you ever wanted to know about the chemistry of zirconia from the Standford Materials Co.

There is another substance that may soon replace cubic zirconium as a cheap diamond substitute. Moissanite, also called carborundum, is a rarely "found" substance (SiC) that is usually dark in color. For many years it has been manufactured for industrial purposes, such as the hard abrasive used in carborundum cutting discs. But a clear version is just now being manufactured by several companies for gemological use. It has similar properties to diamond but has a double refraction at the surface that makes it look a bit fuzzy. Perhaps a perfect gift for your near-sighted loved ones.

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