acoustic wave mowj-e sedâyi Fr.: onde acoustique, ~ sonore A type of → longitudinal wave that consists of mechanical → vibrations of small → amplitude propagated in an → elastic medium. Acoustic waves exhibit phenomena like → diffraction, → reflection, and → interference, but not → polarization. Also called → sonic and → sound waves. See also → acoustic wave equation. The branch of physics concerned with the properties of sonic waves is called → acoustics. |
acoustic wave equation hamugeš-e mowj-e sedâyi Fr.: équation de l'onde acoustique A → differential equation that describes the time evolution of the → scalar potential of the field φ. It is expressed by: ∇^{2}φ = (1/c^{2})∂^{2}φ/∂t^{2}, where c is → velocity of → longitudinal waves and ∇^{2} is the → Laplacian operator. |
advanced wave mowj-e pišras Fr.: onde avancée A wave that travels backward in time according to Maxwell's electromagnetic theory; it arrives before it is transmitted. → Maxwell's equations have two solutions, the normal solution describes the ordinary waves, called → retarded waves, traveling forward in time. However, no advanced waves have ever shown up in any experiment. The advanced solutions of Maxwell's equations are usually simply discarded as "unphysical." Advanced, adj. from advance, → advance of perihelion; → wave. Mowj, → wave; pišras "advanced," from piš "before," Mid.Pers. peš + ras "arriving," from rasidan "to arrive," Mid.Pers. rasitan, O.Pers./Av. rasa- present stem of ar- "to move, go or come toward," cf. Skt. ar-, rcchati. |
Alfvén wave mowj-e Alfvén Fr.: onde d'Alfvén A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave. Named after Hannes Alfvén (1908-1995), Swedish physicist, who developed the theory of → magnetohydrodynamics, and was awarded the Nobel Prize in Physics in 1970; → wave. |
Alfven wave mowj-e Alfvén Fr.: onde d'Alfvén A → magnetohydrodynamic wave in a → magnetized plasma, arising as a result of restoring forces associated with the magnetic field. It is a → transverse wave which propagates in the direction of the magnetic field. Also called magnetohydrodynamic wave. Named after Hannes Alfvén (1908-1995), Swedish physicist, who developed the theory of → magnetohydrodynamics, and was awarded the Nobel Prize in Physics in 1970; → wave. |
ballistic wave mowj-e partâbik Fr.: onde balistique Audible disturbance or wave caused by the compression of air ahead of a projectile in flight. |
blaze wavelength mowjtul-e beliz Fr.: longueur d'onde de blaze The wavelength in a given diffraction order for which the efficiency curve reaches its maximum. → blaze; → wavelength. Mowjtul→ wavelength; beliz→ blaze. |
bow wave farâl-mowj, mowj-e farâl Fr.: onde de proue The wave which appears in front of a speeding boat and goes out behind it in a distinctive "V". It is due to the fact that waves pile up on each other before they can move away. |
center wavelength mowjtul-e markaz Fr.: longueur d'onde centrale Center of a filter passband measured at 50% of peak transmittance. → center; → wavelength. |
central wavelength mowjtul-e markazi Fr.: longueur d'onde centrale 1) In an interference filter, the wavelength of peak
transmission. Central, adj. from → center; → wavelength. |
collapse of the wave function rombeš-e karyâ-ye mowj Fr.: effondrement de la fonction d'onde The idea, central to the → Copenhagen Interpretation of quantum theory, whereby at the moment of observation the → wave function changes irreversibly from a description of all of the possibilities that could be observed to a description of only the event that is observed. More specifically, quantum entities such as electrons exist as waves until they are observed, then "collapse" into point-like particles. According to the Copenhagen Interpretation, observation causes the wave function to collapse. However it is not known what causes the wave function to collapse. Same as → wave collapse. → collapse; → wave function. |
complex wave mowj-e hamtâft (#) Fr.: onde complexe A wave that is composed of several frequencies. |
compression wave mowj-e tanješ Fr.: onde de compression A → longitudinal wave that compresses the → medium along the direction of → propagation, such as a → sound wave. Same as → compressional wave. → compression; → wave. |
compressional wave mowj-e tanješi Fr.: onde de compression An → elastic wave that travels through a → medium with the particles of the medium moving in the same direction as the wave propagation. The compressional wave is the wave that is primarily used in → seismic exploration. Also called P-wave, primary wave, pressure wave. → compressional; → wave. |
Compton wavelength mowjtul-e Compton, tul-e mowj-e ~ Fr.: longueur d'onde de Compton, longueur d'onde Compton The quantum wavelength of a particle with a highly relativistic velocity. The Compton wavelength is given by h/mc, where h is Planck's constant, m is the mass of the particle, and c the light speed. For an electron, the Compton wavelength is about 2.4 × 10^{-10} cm, intermediate between the size of an atomic nucleus and an atom. → Compton; → wavelength. |
cosmic microwave background anisotropy nâhamsângardi-ye tâbeš-e rizmowj-e paszaminé-ye keyhâni Fr.: anisotropies du rayonnement du fond cosmique microonde Tiny fluctuations in the intensity of the → cosmic microwave background radiation (CMBR) as a function of angular position over the sky, first discovered in the → Cosmic Background Explorer (COBE) observations. At a level of 1 part in 100,000, these temperature variations trace the distribution of matter and energy when the Universe was very young, about 380,000 years old. Since the CMB spectrum is described to a high precision by a → blackbody law with temperature T_{0}, it is usual to express the anisotropies in terms of temperature fluctuations ΔT/T_{0} and expand them on the sky in → spherical harmonic series ΔT/T_{0} (θ,φ) = Σ a_{lm}Y_{lm}(θ,φ), where θ and φ are the → spherical polar coordinates, Y_{lm} is the spherical harmonic functions with → multipole index l, and the sum runs over l = 1, 2, ..., ∞, m = -l, ..., l, giving 2l + 1 values of m for each l, and a_{lm} is the multipole moment of the decomposition. The power spectrum of the anisotropies is defined as C_{l}≡ mean | a_{lm} |^{2} = 1/(2l + 1) Σ mean | a_{lm} |^{2}. See also → CMB angular power spectrum. → cosmic; → microwave; → background; → anisotropy. |
cosmic microwave background polarization qotbeš-e zamine-ye rizmowj-e keyhâni Fr.: polarisation du rayonnement du fond cosmique microonde The polarization of the → cosmic microwave background radiation due to → Thomson scattering by → free electrons during the → recombination era. The polarization can greatly enhance the precision with which the parameters associated with → acoustic oscillations are derived; because it carries directional information on the sky. When an → electromagnetic wave is incident on a free electron, the scattered wave is polarized perpendicular to the incidence direction. If the incident radiation were → isotropic or had only a → dipole variation, the scattered radiation would have no net polarization. However, if the incident radiation from perpendicular directions (separated by 90°) had different intensities, a net → linear polarization would result. Such → anisotropy is called → quadrupole because the poles of anisotropy are 360°/4 = 90° apart. → cosmic; → microwave; → background; → polarization. |
cosmic microwave background radiation (CMBR) tâbeš-e rizmowj-e paszaminé-ye keyhâni Fr.: rayonnement du fond cosmique microonde The diffuse → electromagnetic radiation in the → microwave band, coming from all directions in the sky, which consists of relic photons left over from the very hot, early phase of the → Big Bang. More specifically, the CMBR belong to the → recombination era, when the → Universe was about 380,000 years old and had a temperature of about 3,000 K, or a → redshift of about 1,100. The photons that last scattered at this epoch have now cooled down to a temperature of 2.73 K. They have a pure → blackbody spectrum as they were at → thermal equilibrium before → decoupling. The CMB was discovered serendipitously in 1965 by Penzias and Wilson (ApJ L 142, 419) and was immediately interpreted as a relic radiation of the Big Bang by Dicke et al. (1965, ApJL 142, 383). Such a radiation had been predicted before by Gamow (1948, Nature 162, 680) and by Alpher and Herman (1948, Nature 162, 774). This discovery was a major argument in favor of the Big Bang theory. In 1992, the satellite → Cosmic Background Explorer (COBE) discovered the first anisotropies in the temperature of the CMB with an amplitude of about 30 µK. See also: → cosmic microwave background anisotropy, → dipole anisotropy, → CMB lensing, → CMB angular power spectrum, → acoustic peak, → baryon acoustic oscillation, → WMAP. → cosmic; → microwave; → background; → radiation. |
cutoff wavelength mowj-tul-e boré Fr.: longueur d'onde de coupure Wavelength at which the transmittance of a filter, or the detectivity of a detector, has fallen to one-half its peak value. → cutoff; → wavelength. |
de Broglie wavelength mowjtul-e de Broglie Fr.: longueur d'onde de Broglie The wavelength of the wave associated with a → particle as given by the → de Broglie equation. |