![]() In general, an index of refraction is a complex number with both a real and imaginary part, where the latter indicates the strength of absorption loss at a particular wavelength-thus, the imaginary part is sometimes called the extinction coefficient k " (PDF). Thus, it is especially important to cite the source for an index measurement if precision is required. There are also weaker dependencies on temperature, pressure/ stress, etc., as well on precise material compositions (presence of dopants, etc.) for many materials and typical conditions, however, these variations are at the percent level or less. Standard refractive index measurements are taken at the "yellow doublet" sodium D line, with a wavelength of 589 nanometers. Many materials have a well-characterized refractive index, but these indexes often depend strongly upon the frequency of light, causing optical dispersion. ![]()
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