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In 1834, Charles Wheatstone developed a method of using a rapidly rotating mirror to study transient phenomena, and applied this method to measure the velocity of electricity in a wire and the duration of an electric spark. [1] He communicated to François Arago the idea that his method could be adapted to a study of the speed of light.
Fizeau used a special interferometer arrangement to measure the effect of movement of a medium upon the speed of light. According to the theories prevailing at the time, light traveling through a moving medium would be dragged along by the medium, so that the measured speed of the light would be a simple sum of its speed through the medium plus ...
Spectroscopy, primarily in the electromagnetic spectrum, is a fundamental exploratory tool in the fields of astronomy, chemistry, materials science, and physics, allowing the composition, physical structure and electronic structure of matter to be investigated at the atomic, molecular and macro scale, and over astronomical distances.
Physics theories of the 19th century assumed that just as surface water waves must have a supporting substance, i.e., a "medium", to move across (in this case water), and audible sound requires a medium to transmit its wave motions (such as air or water), so light must also require a medium, the "luminiferous aether", to transmit its wave ...
At the behest of the Paris Observatory under Urbain Le Verrier, Marie Alfred Cornu repeated Fizeau's 1848 toothed wheel measurement in a series of experiments from 1872 to 1876. The goal was to obtain a value for the speed of light accurate to one part in a thousand.
Phase velocity, the velocity at which a wave phase propagates; Pulse wave velocity, the velocity at which a pulse travels through a medium, usually applied to arteries as a measure of arterial stiffness; Group velocity, the propagation velocity for the envelope of wave groups and often of wave energy, different from the phase velocity for ...
As a specific example in which the unique features of Laser Doppler velocimetry are important, the measurement of velocity of a MEMS watt balance device [14] has allowed greater accuracy in the measurement of small forces than previously possible, through directly measuring the ratio of this velocity to the speed of light. This is a fundamental ...
The phase velocity of X-rays through most glasses can routinely exceed c, [45] but phase velocity does not determine the velocity at which waves convey information. [ 46 ] If a laser beam is swept quickly across a distant object, the spot of light can move faster than c , although the initial movement of the spot is delayed because of the time ...