The speed of light in a vacuum (299,792,458 m/s) is one of the fundamental constants of nature. The speed of light in air is essentially the same. It is convenient to refer to the speed of light in other (“denser”) materials by comparison to the speed of light in a vacuum: the ratio of the speed of light in a vacuum to the speed of light in another medium is referred to as the refractive index of the medium. (This casual use of the term denser has nothing to do with the specific gravity of different materials.) We abbreviate the refractive index as n,
and use subscripts to designate different media as appropriate. Because the speed of light in a vacuum is always greater than its speed in any material medium, the refractive index of any material medium is always greater than 1.0.
A material’s chemical composition, and sometimes other factors, strongly influence its refractive index. By incorporating small amounts of various additives, manufacturers can vary the refractive index (and other properties) of optical glass from less than 1.4 to more than 1.9. The refractive indices of various materials of interest are listed in Table 1-1.
Although it is occasionally critical to determine the refractive index to several decimal places, approximate values are sufficiently accurate for most clinical purposes. For example, the refractive index of the vitreous humor is 1.336, but the approximate value of
is much easier to remember and introduces negligible error.
Table 1-1 Refractive Indices of Some Clinically Important Mediaa
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.