Light that travels through an optical material is slowed by its interaction with that material. So, when there is no interaction with any material, as in a vacuum, light travels at its maximum velocity. This velocity is a constant and is approximately 186,000 miles per second or about a nanosecond (0.000000001 second) per foot. This is the maximum rate at which information can be transmitted. To put this in perspective, it takes light 16 milliseconds (0.016 sec) to travel the 2,912 miles from Seattle to New York. In comparison, a jet passenger plane flies about 600 miles per hour, or .17 miles per second, over a million times slower.
A fundamental property of any optical material is its index of refraction. This number is the ratio of the velocity of light in a vacuum divided by the velocity of light in the optical medium. Since the velocity of light in any material medium is always is less than it's vacuum velocity, this number is always positive and larger than one (There is current research in building specially fabricated optical structures that create a negative index). The index of refraction of a material is also dependent on the wavelength of the light. A material could have a different value with red light versus blue light. A materials dispersion is just this difference between the index at the long wavelength red versus the short wavelength blue. For people gesigning lenses, dispersion is the cause of chromic aberation, of the focusing of different color at different places. Optical materials that have low dispersion have a relative flat curve of their index versus the wavelength of light, such as acrylic. Hight dispersion materials are materials such as lead glass or diamond (.044).
| The index of refraction for relevant optical materials | |
|---|---|
| Air (at STP) | 1.00029 |
| Water | 1.33 |
| Quartz | 1.54 |
| Acrylic | 1.49 |
| Optical Glass (depending on composition) | 1.5 to 1.8 |
| Sapphire | 1.77 |
| Diamond | 2.42 |
The density is the weight of a material per unit volume. As can be seen for instance, glass is twice as heavy as acrylic for the same amount of material. The relative density of an optical material is the ratio of the density of the material divided by the density of water.
| The relative density of optical materials | |
|---|---|
| Water | 1.0 |
| Quartz | 2.65 |
| Acrylic | 1.19 |
| Glass | 2.3 - 5.1 |
| Sapphire | 3.4 |
| Diamond | 3.5 |
Relative hardness of optical materials gauges their relative scratch resistance.
| The relative hardness of optical materials | |
|---|---|
| Quartz | 7 |
| Acrylic | 3 |
| Glass | 4 - 5 |
| Sapphire | 9 |
| Diamond | 10 |
The elemental make-up of these optical materials is another way to compare these substances. The study of Chemistry divides all materials of the world into compounds that are made from Carbon (Organic Chemistry) and all the rest (Inorganic Chemistry). The most common elements in the earth's crust by weight are 46.6% Oxygen, 27.7% Silicon, 8.1% Aluminum, 5% Iron , 3.6% Calcium, 2.8% Sodium, 2.6% Potassium, and 2.1% Magnesium. Therefore, over 74% of the earths crust is oxygen and silicon, adding the other six elements accounts for 99% of the earths crust weight. Carbon by comparison is .03% by wieght of the earth's crust
Although Carbon forms only a small percentage of the earths crust, it forms the basis of all plant and animal life. Carbon chemistry is the foundation of all polymers, rubbers, wood, gases like methane, ethane, propane and gasoline to name a few. Amino acids, nucleotides (DNA, RDA), lipids, and sugars are carbon based. It is well known that diamond is pure carbon, bonded in a tetrahedral structure and that graphite is also pure carbon, bonded in a multiple sheet structure. Acrylic is a carbon based polymer with oxygen and hydrogen.
Quartz is a pure crystal of silicon dioxide (silica). Glasses are considered to be amorphous solid materials, not chemical compounds. Soda lime glasses are the most common and are composed of 60%-70% silica, 12%-18% soda (Na20) and 5%-12% lime (Calcium oxide). Flint glass or Lead glass has at least 20% lead oxide to increase their dispersion, and has a relatively soft surface. Borosilicate glasses have at least 5% Boric oxide which makes them more resistance to thermal shock and chemical corrosion. Opals are silica materials where microscopic spheres of silica are closly packed in a three dimensitional structure to produce a natural diffraction grating that creates the colors. Other names for colored quartz are amethyst, citrine, ametrine, rose quartz and agate. Each of these is pure silica with trace metallic elements that produce the different colors. These trace metallic elements are usually transition metal elements (copper, chromium, manganese, iron, cobalt, nickel, vanadium and titanium) or rare earth elements, mainly neodymium and praseodymium.
Other than diamond, most gemstones are metal oxides. Aluminum oxides make up the Corundum family, which include Ruby and Sapphire. Trace metallic elements give the different colors to these minerals.