respect to another reference medium 1 (n21) is given by the ratio of speed of light in medium 1 to that in medium 2. This can be expressed as follows: If the reference medium 1 is vacuum, then the refractive index of medium 2 is considered with respect to vacuum. It is simply represented as n2 and is called the absolute refractive index of medium 2. The absolute refractive index n of an optical medium is defined as the ratio of the speed of light in vacuum, , and the phase velocity v of light in the medium, Since c is constant, n is inversely proportional to v : The phase velocity is the speed at which the crests or the phase of the wave moves, which may be different from the group velocity, the speed at which the pulse of light or the envelope of the wave moves. Historically air at a standardized pressure and temperature has been common as a reference medium. History Thomas Young was presumably the person who first used, and invented, the name "index of refraction", in 1807. At the same time he changed this value of refractive power into a single number, instead of the traditional ratio of two numbers. The ratio had the disadvantage of different appearances. Newton, who called it the "proportion of the sines of incidence and refraction", wrote it as a ratio of two numbers, like "529 to 396" (or "nearly 4 to 3"; for water). Hauksbee, who called it the "ratio of refraction", wrote it as a ratio with a fixed numerator, like "10000 to 7451.9" (for urine). Hutton wrote it as a ratio with a fixed denominator, like 1.3358 to 1 (water). Young did not use a symbol for the index of refraction, in 1807. In the later years, others started using different symbols: n, m, and µ. The symbol n gradually prevailed. Typical values Refractive index also varies with wavelength of the light as given by Cauchy's equation: The most general form of Cauchy's equation is where n is the refractive index, λ is the wavelength, A, B, C, etc., are coefficients that can be determined for a material by fitting the equation to measured refractive indices at known wavelengths. The coefficients are usually quoted for λ as the vacuum wavelength in micrometres. Usually, it is sufficient to use a two-term form of the equation: where the