Basic Optical Principles
Index of Refraction

Material	Index
Air	1.000
Cornea	1.370
Crystalline Lens	1.420
Vitreous Humor	1.333
Opthalmic Crown Glass	1.523
CR-39 Plastic	1.490
Polycarbonate	1.586
Barium Glass	1.602
Flint Glass 1.8	1.805
Titanium Glass	1.701

The index of refraction of any given material is calculated by dividing the speed of light in air by the speed of light in that material. This can be expressed in the following formula:

n =	speed of light in air
	speed of light in material

 

 

where n is the mathematical symbol which indicates index of refraction.

Therefore, if a given material such as CR-39 plastic has an index of refraction of 1.490, it can be calculated that the speed of light in this material=124,832 miles per second, or 1.490 times slower than it travels in air.

The standard index of refraction used in the optical industry is 1.530. Although there is no ophthalmic material commonly used with this index, most optical equipment and tools such as lens clocks and surfacing laps, are calibrated to this index. When materials with other indices are used, calculations are made to �compensate" for their varying densities.