The phase velocity v of a wave in a given uniform medium is given by However, in lenses, dispersion causes chromatic aberration, an undesired effect that may degrade images in microscopes, telescopes, and photographic objectives. The dispersion of light by glass prisms is used to construct spectrometers and spectroradiometers. Material dispersion can be a desirable or undesirable effect in optical applications. Influences of selected glass component additions on the mean dispersion of a specific base glass ( n F valid for λ = 486 nm (blue), n C valid for λ = 656 nm (red)) The wavelengths of visible light are shaded in grey. Material dispersion in optics The variation of refractive index vs. For example, in fiber optics the material and waveguide dispersion can effectively cancel each other out to produce a zero-dispersion wavelength, important for fast fiber-optic communication. In a waveguide, both types of dispersion will generally be present, although they are not strictly additive. More generally, "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure (e.g., a photonic crystal), whether or not the waves are confined to some region. However, in a waveguide there is also the phenomenon of waveguide dispersion, in which case a wave's phase velocity in a structure depends on its frequency simply due to the structure's geometry.
Most often, chromatic dispersion refers to bulk material dispersion, that is, the change in refractive index with optical frequency. However, dispersion also has an effect in many other circumstances: for example, group-velocity dispersion causes pulses to spread in optical fibers, degrading signals over long distances also, a cancellation between group-velocity dispersion and nonlinear effects leads to soliton waves. The most familiar example of dispersion is probably a rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths (different colors). crimped segments in a cable) can produce signal distortion which further aggravates inconsistent transit time as observed across signal bandwidth. In some applications such as telecommunications, the absolute phase of a wave is often not important but only the propagation of wave packets or "pulses" in that case one is interested only in variations of group velocity with frequency, so-called group-velocity dispersion.Īll common transmission media also vary in attenuation (normalized to transmission length) as a function of frequency, leading to attenuation distortion this is not dispersion, although sometimes reflections at closely spaced impedance boundaries (e.g. Design of compound achromatic lenses, in which chromatic aberration is largely cancelled, uses a quantification of a glass's dispersion given by its Abbe number V, where lower Abbe numbers correspond to greater dispersion over the visible spectrum. In optics, one important and familiar consequence of dispersion is the change in the angle of refraction of different colors of light, as seen in the spectrum produced by a dispersive prism and in chromatic aberration of lenses. Within optics, dispersion is a property of telecommunication signals along transmission lines (such as microwaves in coaxial cable) or the pulses of light in optical fiber. In optics and in wave propagation in general, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency sometimes the term chromatic dispersion is used for specificity to optics in particular.Ī medium having this common property may be termed a dispersive medium (plural dispersive media).Īlthough the term is used in the field of optics to describe light and other electromagnetic waves, dispersion in the same sense can apply to any sort of wave motion such as acoustic dispersion in the case of sound and seismic waves, and in gravity waves (ocean waves). A compact fluorescent lamp seen through an Amici prism Dependence of phase velocity on frequency In a dispersive prism, material dispersion (a wavelength-dependent refractive index) causes different colors to refract at different angles, splitting white light into a spectrum.
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