Velocity of light in vacuum

The refractive index of a medium or a compound, n, is defined as c/v, the ratio of the velocity of light in vacuum (c) to the velocity of light in the medium or compound (v). Reported values usually refer to the ratio of the velocity in air to that in the air-saturated compound [1]. If the light of the sodium D line (wavelength / = 589.3 nm) is used at temperature t(°C), the measured refractive index is denoted n D. 

The refractive index (or index of refraction) of a medium is defined as a ratio of the velocity of light in vacuum over the velocity of light in the substance of interest (a medium), or, in other words, is a measure for how much the speed of light (or other waves such as sound waves) is reduced inside the medium. For example, typical glass has a refractive index of 1.5, which means that light travels at 1/1.5 =0.67 times the speed in air or vacuum. 

The velocity of light in vacuum is the same for all wavelengths, 2.998 x 10 m/s, so that a simple inverse proportionality exists between wavelength and frequency. The velocity of light in the atmosphere is reduced by less than one part in a thousand below the value for vacuum, so that the above value is acceptable for most laboratory applications. Another term is often used, the wave number, />, defined as 1/A, or as v/c. The most common unit for wave number is cm ". ... 

In planar cavities, the optical modes are discrete and the frequencies of these modes are integer multiples of the fundamental mode frequency, as shown schematically in Fig. 1.7. The fundamental and first excited mode occur at frequencies of vQ and 2v0, respectively. For a cavity with two metallic reflectors (no distributed Bragg reflectors) and a jt phase shift of the optical wave upon reflection, the fundamental frequency is given by v0 = c / 2nlcaw where c is the velocity of light in vacuum and Icav is the length of the cavity. In a resonant cavity, the emission frequency of an optically active medium located inside the cavity equals the frequency of one of the cavity modes. 

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. This definition is based on the constant velocity of light in vacuum at exactly 299792458 m s . ... 

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