![]() ![]() Besides, there are optical frequency standards relying on far more sophisticated technology, where the obtained wavelength is by many orders of magnitude more precise. It is common to use certain spectral lamps as reasonably accurate wavelength standards. The indirect evaluation of the wavelength from a measured optical frequency and the vacuum velocity of light can be far more precise, since optical frequencies can be measured with extremely high precision, and the vacuum velocity of light is nowadays (in the SI system) a defined quantity, i.e., without any measurement error. Optical wavelengths can be measured with wavemeters, which are a kind of interferometers. ![]() For the conversion between wavelength and frequency ranges one needs to keep in mind that the width of a frequency interval does not only depend on the width of the corresponding wavelength interval but also on the mean wavelength: (assuming small intervals). If some physical quantity depends on an optical frequency, it is very often called wavelength-dependent instead of frequency-dependent, even if the spatial aspect does not play a role in the relevant phenomena.Īlso one often specifies an optical bandwidth (e.g., the gain bandwidth of a laser gain medium) in terms of the width of the wavelength range rather than the frequency range. Light with longer vacuum wavelengths is called infrared light, while light with shorter wavelengths is ultraviolet light. Generally, the refractive index depends on the optical frequency or vacuum wavelength (→ chromatic dispersion).įor visible light, the vacuum wavelength is roughly between 400 nm and 700 nm there are no precisely defined boundaries of the visible spectral region, since the sensitivity of the human eye is a smooth function of wavelength and also differs between individuals. Wavelength calculations Vacuum wavelength:Įnter input values with units, where appropriate.Īfter you have modified some values, click a “calc” button to recalculate the field left of it.Īttention: The buttons do not work, as Javascript is turned off in your browser!įor a given vacuum wavelength, the wavelength in a medium with refractive index is. For example, a vacuum wavelength of 1000 nm would correspond to 999.7259 nm in air. The wavelength in air is quite close to the vacuum wavelength, since the refractive index of air is only very slightly above 1 the small difference is not relevant for most applications. In some cases, however, the vacuum wavelength is specified. For historical reasons, it is more common, however, to characterize optical waves ( light) with their wavelength in air (for standard pressure and temperature: 1013.25 mbar, 15 ☌, zero humidity). ![]() Therefore, it would be most natural to characterize such a wave by its optical frequency. If a monochromatic optical wave subsequently propagates through different transparent media, its wavelength will vary, while its optical frequency stays constant. Optical Waves: Vacuum Wavelength and the Wavelength in a Medium There are objective quantitative measures for quantities related to color perception see the article on colorimetry. Figure 2: Approximate colors for different wavelengths. This has been done in Figure 1, which will appear somewhat differently on different computer screens. Unfortunately, a computer screen cannot produce monochromatic light, but only roughly approximate the visual impression for certain wavelength. Monochromatic light is seen with a certain color depending on its wavelength. See also the article on the Gouy phase shift, which is essentially a phase deviation of Gaussian beams against a plane wave for comparison. Note that for waves with a different spatial distribution – for example, for strongly focused laser beams, the amplitude is a function of position is in general not (or not exactly) periodic, and the spacing between subsequent wave maxima may somewhat deviate from the wavelength, which is always defined for a plane wave. Wavelength and frequency are related to each other: as the wave propagates by one wavelength within one oscillation cycle, its phase velocity is given by:įor wave packets, there is another kind of velocity, called the group velocity, which can deviate from the phase velocity if there is chromatic dispersion, i.e., a dependence of the phase velocity on the frequency. When propagating by one wavelength in direction, the plane wave acquires a phase delay of. ![]() (Note that in spectroscopy the wavenumber is normally considered as the inverse of the wavelength, not involving the factor. $$A(\vec r,t) = $$įigure 1: A plane wave, with the wavelength illustrated with the black ruler in the middle. The simplest kind of wave is a monochromatic plane wave, described by the following complex wave amplitude, being a function of spatial position and time : ![]()
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