An Etymological Dictionary of Astronomy and Astrophysics
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The → Zeeman effect when the emitting source is viewed in the direction of the magnetic field. In the normal longitudinal effect, each spectral line is split into two components with frequencies ν ± Δν. The line with the frequency ν - Δν shows left-hand → circular polarization and that with frequency ν + Δν shows right-hand circular polarization. → transverse Zeeman effect.

→ longitudinal; → Zeeman effect.

The resonant and recoil-free emission and absorption of gamma rays by atoms bound in a solid form.

Named after Rudolf Mößbauer (1929-), a German physicist who studied gamma rays from nuclear transitions, and discovered this phenomenon in 1957; → effect.

Reduced growth or stagnation undergone by → cold dark matter perturbations during the period when the → early Universe was → radiation-dominated. The photons cannot collapse, and by their pressure prevent the matter to do so, when radiation dominates. Matter pertubation (collapse) remains frozen until the density equality between radiation and matter.

Péter Mészáros, 1974, A&A 37, 225; → effect.

When a temperature gradient is maintained through a strip of metal in a magnetic field, the direction of flow being across the lines of force, a potential difference will be produced across the conductor.

Walter Nernst (1864-1941), German physical chemist; → effect.

Oskar, → effect.

A feature appearing in an observation, which is not intrinsic to the object observed, but is due to the inappropriate method used (e.g. limited angular resolution).

→ observational; → effect.

In the → solar dynamo model, the process whereby the → meridional magnetic field is stretched into an → azimuthal magnetic field by → differential rotation. See also → alpha effect.

Omega (ω), Gk. letter of alphabet; → effect.

An effect on spectral lines obtained when the light source is located in a strong magnetic field. The strong field disrupts the coupling between the orbital and spin angular momenta, resulting in a different pattern of splitting.

Named for the German physicists Friedrich Paschen (1865-1947) and Ernst E. A. Back (1881-1959); → effect.

When an electric current is sent through the junction between two different conductors or semiconductors, a quantity of heat is liberated or absorbed, depending on the direction of the current. The heat is proportional to the total electric charge crossing the junction. This effect is due to the existence of an electromotive force at the junction.

Named after Jean-Charles Peltier (1785-1845), French physicist and watchmaker, who discovered the effect in 1834; → effect.

The process of release of electrically charged particles (usually → electrons) as a result of irradiation of matter by light or other → electromagnetic radiation. The classical electromagnetic theory was unable to account for the following characteristics of the phenomenon. Light below a certain threshold frequency, no matter how intense, will not cause any electrons to be emitted. Light above that frequency, even if it is not very intense, will always cause electrons to be ejected. The electrons are ejected after some nanoseconds, independently of the light intensity. The maximum kinetic energy of the emitted electrons is a function of the frequency and does not dependent on the intensity of the incident light. The classical theory could not explain how a train of light waves spread out over a large number of atoms could, in a very short time interval, concentrate enough energy to knock a single electron out of the metal. In 1905, based on Planck's idea of → quanta, Einstein proposed that light consisted of quanta (later called → photons); that a given source could emit and absorb radiant energy only in units which are all exactly equal to the radiation frequency multiplied by a constant (→ Planck's constant); and that a photon with a frequency over a certain threshold would have sufficient energy to eject a single electron. His photoelectric equation is descibed as (1/2)mu² = hν - A, where m is the electron mass, u is the electron velocity, h is Planck's constant, ν is the frequency, and A the → work function, which represents the amount of work needed by electrons to get free of the surface. See also → photoelectron, → photoelectric current, → external photoelectric effect, → internal photoelectric effect.

→ photoelectric; → effect.

The emission of electrons as a result of incident radiation in the → photoelectric effect. Also called → external photoelectric effect.

→ photo- + → emissive; → effect.

The property exhibited by some crystals (notably quartz) that develop an electric charge or potential difference across them when subjected to mechanical strain; and conversely produce mechanical forces when a voltage is applied to them in a suitable manner.

From piezo-, from Gk. piezein "to press tight" + → electric; → effect.

oskar, → effect; fešârbarqi pertaining to fešârbarq, from fešâr, → pressure, + barq, → electricity.

The effect of → solar radiation on a small (centimeter-sized) particle in → orbit around the Sun that causes it to lose velocity and fall gradually into the Sun. The particle → absorbs solar radiation and → radiates the energy → isotropically in its own frame. The particle thereby preferentially radiates (and loses → angular momentum) in the forward direction in the → inertial frame of the Sun (aberration effect). This leads to a decrease in the particle's angular momentum and causes it to spiral sunward. In contrast, the → Yarkovsky effect is anisotropic; the object may be accelerated or decelerated.

→ Poynting's theorem; Howard Percy Robertson (1903-1961), American physicist and mathematician; → effect.

The increasing sensibility of the retina for light of shorter wavelength as the brightness decreases. In those conditions red objects are perceived to fade faster than blue objects of the same brightness.

Named after the Czech anatomist Jan Evangelista Purkynne (1787-1869), who discovered the effect; → effect.

Same as → Raman scattering.

Named after the Indian physicist Sir Chandrasekhara Venkata Raman (1888-1970), who discovered the effect; recipient of the 1930 Nobel Prize in Physics; → effect.

The → Sachs-Wolfe effect in which the calculations are extended to nonlinear mass concentrations. In the non-linear regime of large-scale → structure formation the → gravitational potential changes with time, and photons climb out of a → potential well slightly different from the one that they fell into. Therefore, nonlinear density fluctuations produce extra evolution of the potentials against the background expansion. On large scales, the nonlinear contribution to the full ISW effect is expected to be dominated by the linear ISW effect in a Universe with → cosmological constant (Seljak, 1996, ApJ 460, 549).

Martin J. Rees (1942-) & Dennis W. Sciama (1926-1999), 1968, Nature 217, 511; → effect.

The Doppler effect when the relative motion of the source and the observer is comparable to the speed of light. In that case the classical Doppler formula should be corrected for effects of the special theory of relativity (Lorentz transformation).

→ relativistic; → Doppler effect.

A physical phenomenon described only by the theories of → special relativity or → general relativity.

→ relativistic; → effect.

A → spectroscopic phenomenon observed when either an → eclipsing binary's → secondary star or an → extrasolar planet is seen to → transit across the face of the → primary body. Because of the rotation of the star, an asymmetric distortion takes place in the → line profiles of the stellar spectrum, which changes during the transit. The measurement of this effect can be used to derive the → alignment of the → orbit of the transiting exoplanet with respect to the → rotation axis of the star.

Named after Richard Alfred Rossiter (1886-1977) and Dean Benjamin McLaughlin (1901-1965), American astronomers.

The effect of → gravitational potentials on the → anisotropy of the → cosmic microwave background radiation, in which photons from the → CMB are gravitationally → redshifted, causing the CMB spectrum to appear uneven. This effect is the predominant source of fluctuations in the CMB for angular scales above about 10 degrees. It involves two parts: the effect of the potential at the → surface of last scattering, which is the ordinary Sachs-Wolfe effect. And the integrated Sachs-Wolfe (ISW) effec, which is caused by the time variation of gravitational potentials as the photons travel through them. A photon traveling through a decaying → potential well (wall) gains (loses) energy. Without → dark energy the photon is → blueshifted and then → redshifted, so that both effects compensate each other. On the other hand, in an → accelerating Universe driven by dark energy the photon gets more blueshifted. See also → Rees-Sciama effect.

Rainer Kurt Sachs (1932- ) & Arthur Michael Wolfe (1939- ), 1967, ApJ 147, 73; → effect.

The → phase difference between two light waves moving in opposite directions along a closed circular loop when the loop is rotating. More specifically, consider a beam of light split into two beams which are then allowed to propagate in two opposite directions along the rim of a rotating disk. When they are recombined, a phase difference occurs between them. The position of the → interference fringes is dependent on the → angular velocity of the setup. This → relativistic effect illustrates the impossibility of synchronizing clocks situated in a rotating → reference frame, as described by Einstein in 1905. The Sagnac effect is used, for example, in optical gyroscopes installed in airplanes or in devices used for measuring the Earth rotation. The Sagnac effect is very important for the correct working of the → Global Positioning System.

Named after Georges Sagnac (1869-1928), French physicist, who discovered the phenomenon in 1913; → effect.