dispersion

dispersion, in chemistry

dispersion, in chemistry, mixture in which fine particles of one substance are scattered throughout another substance. A dispersion is classed as a suspension, colloid, or solution. Generally, the particles in a solution are of molecular or ionic size; those in a colloid are larger but too small to be observed with an ordinary microscope; those in a suspension can be observed under a microscope or with the naked eye. A coarse mixture (e.g., sand mixed with sugar) is usually not thought of as a dispersion.

---

dispersion, in physics

dispersion, in physics: see spectrum.

---

dispersion

Any phenomenon associated with the propagation of individual waves at speeds that depend on their wavelengths. Wavelength determines the speeds at which waves travel through media. This variation in speed causes radiation to separate into components that have different frequencies and wavelengths. For example, when a beam of white light is sent through a glass prism, refraction causes the beam to disperse into an array of its component colours of light, producing a rainbowlike effect.

---

dispersion

In optical fibers, the broadening of the waveforms over long distances by the time they reach the receiving end, which makes them difficult to interpret. There are three major causes. One is the multiple transmission paths (modes) possible in large-core multimode fibers where each path results in a different travel distance.

A second has to do with the varying of the refractive index due to changes in frequency (or correspondingly, changes in wavelength). The speed of light in a fiber is based on the frequency of light and the refractive index of the fiber. Thus, different frequencies travel at different speeds. The problem is that there are always multiple frequencies. Analog signals are naturally many frequencies, but digital pulses are also more than one frequency, because it is difficult to create a perfect single frequency.

The third has to do with the random fluctuations of light polarization inside the fiber. Following are the common types of dispersion. See refractive index, dispersion compensator, step index fiber, graded-index fiber, dispersion-shifted fiber and fiber optics glossary.

Modal dispersion (or intermodal dispersion)
Occurs in multimode fibers, because light travels in multiple modes (reflective paths), and each path results in a different travel distance. Modal dispersion is a major problem with multimode fibers.

Chromatic dispersion
The sum of material dispersion and waveguide dispersion. "Material dispersion" is caused by the variation in refractive index of the glass in the fiber. "Waveguide dispersion" is due to changes in the distribution of light between the core and the cladding of a singlemode fiber.

Polarization mode dispersion (PMD)
Light travels in two polarization states in singlemode fibers. Over long distances, conditions such as stress and slight irregularities in the fiber core cause random fluctuations in how the two polarizations travel through the fiber. As a result, they gradually spread over the square root of the distance.

---

dispersion [də′spər·zhən]

(aerospace engineering)

Deviation from a prescribed flight path; specifically, circular dispersion especially as applied to missiles.

(astronomy)

The frequency dependence of the retardation of radio waves (such as those emitted by a pulsar) when they pass through an ionized gas.

(chemistry)

A distribution of finely divided particles in a medium.

(communications)

The entropy of the output of a communications channel when the input is known.

(electromagnetism)

Scattering of microwave radiation by an obstruction.

(mineralogy)

In optical mineralogy, the constant optical values at different positions on the spectrum.

(physics)

The separation of a complex of electromagnetic or sound waves into its various frequency components.

Quantitatively, the rate of change of refractive index with wavelength or frequency at a given wavelength or frequency.

The rate of change of deviation with wavelength or frequency.

In general, any process separating radiation into components having different frequencies, energies, velocities, or other characteristics, such as the sorting of electrons according to velocity in a magnetic field.

(statistics)

The degree of spread shown by observations in a sample or a population.

---

Dispersion (radiation)

The separation, by refraction, interference, scattering, or diffraction, of acoustic and electromagnetic radiation or energy into its constituent wavelengths or frequencies. For a refracting, transparent substance, such as a prism of glass, the dispersion is characterized by the variation of refractive index with change in wavelength of the radiation. Refractive index (n) is defined as the ratio of the velocity of the radiation in free space (air at standard temperature and pressure for sound, and a vacuum for electromagnetic radiation) to the velocity in the substance in question. I. Newton used a small hole in a window shade and a glass prism to disperse sunlight into a visible spectrum, from violet through red. Using a second prism, he showed that no further decomposition of any of the spectral colors could be achieved. See Optical prism, Refraction of waves

The condition where the refractive index decreases as wavelength increases is termed normal dispersion. The opposite condition is termed anomalous dispersion, and almost always occurs in regions outside the range of visible wavelengths.