The Speed of Light

How do we know that light has a finite speed and does not move infinitely fest  

During the late 1600s, the Danish astronomer Ole Romer (1644-1710) measured the orbits of several of Jupiter s moons. These moons move much faster than our own—they have orbits of 1-7 days and are eclipsed by Jupiter s shadow at every revolution. Over many months, Romer measured discrepancies of up to 10 minutes in the times of these orbits. He reasoned that the discrepancies occurred because Jupiter was farther from Earth at different times of the year. Thus, light from the Sun, which reflected off Jupiter and ultimately to his telescope, had farther to travel at different times of the year, implying that light travels at a finite speed. Romer s data led to the first estimate of the speed of light, 3.5 X 10 m/s. 

By 1975, the measured value was even more precise, 2.99792458 0.00000004 X 10 m/s (in vacuum), the error being mostly due to the uncertainty in the length of the meter. In 1983, the meter was redefined based on the distance that hght travels in vacuum in one second. As a result, the value for the speed of hght became a fixed, exact quantity, c = 2.99792458 X 10 m/s. 

The yellow light given off by a sodium vapor lamp used for public lighting has a wavelength of 589 nm. What is the frequency of this radiation  

Analyze We are given the wavelength. A, of the radiation and asked to calculate its frequency, v. 

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At still shorter time scales other techniques can be used to detenuiue excited-state lifetimes, but perhaps not as precisely. Streak cameras can be used to measure faster changes in light intensity. Probably the most iisellil teclmiques are pump-probe methods where one intense laser pulse is used to excite a sample and a weaker pulse, delayed by a known amount of time, is used to probe changes in absorption or other properties caused by the excitation. At short time scales the delay is readily adjusted by varying the path length travelled by the beams, letting the speed of light set the delay. 

The detector D monitors the absorption of the probe beam as a function of the delay between the pulses given by xHc, where c is the speed of light and v is the difference between the optical path travelled by the probe and by the pump pulse. Adapted from [110],... 

To get the frequency v in centimeters-, the nonstandard notation favored by spectioscopists, one divides the frequency in hertz by the speed of light in a vacuum, c = 2.998 x lO " cm s-, to obtain a reciprocal wavelength, in this case, 4120 cm-. This relationship arises because the speed of any running wave is its frequency times its wavelength, c = vX in the case of electromagnetic radiation. The Raman spectral line for the fundamental vibration of H2 is 4162 cm-. .., not a bad comparison for a simple model. 

Electromagnetic radiation (Section 13 1) Vanous forms of ra diation propagated at the speed of light Electromagnetic radiation includes (among others) visible light infrared ul traviolet and microwave radiation and radio waves cos mic rays and X rays... 

Quantum mechanical calculations generally have only one carbon atom type, compared with the many types of carbon atoms associated with a molecular mechanics force field like AMBER. Therefore, the number of quantum mechanics parameters needed for all possible molecules is much smaller. In principle, very accurate quantum mechanical calculations need no parameters at all, except fundamental constants such as the speed of light, etc. 

In the second version of this equation c is the speed of light, and X the wavelength of the radiation. 

In a vacuum all electromagnetic radiation travels at the same speed, the speed of light c, and may be characterized by its wavelength X, in air or vacuum, or by its wavenumber v or frequency v, both conventionally in a vacuum, where... 

In the this form the Beer-Lambert law shows that the intensity of radiation transmitted by an absorbing sample declines exponentially as the length over which the absorption takes place increases. If the radiation, travelling with the speed of light c, takes time tg to traverse the absorbing path f Equation (9.29) becomes ... 

Free-Electron Lasers. The free-electron laser (EEL) directly converts the kinetic energy of a relativistic electron beam into light (45,46). Relativistic electron beams have velocities that approach the speed of light. The active medium is a beam of free electrons. The EEL, a specialized device having probably limited appHcations, is a novel type of laser with high tunabiHty and potentially high power and efficiency. 

Spaceships capable of reaching stars other than the sun are expected to be more directly involved with plasmas than are contemporary spacecraft, in terms of their motion through the interstellar plasmas and their propulsion. Very high velocities are expected to be required for travel to other stars, eg, Proxima Centauri, which is 4.3 light years distant and would require 43 years at one-tenth the speed of light. 

In order to increase the precision of realization of the base unit meter, the definition based on the wavelength of a krypton-86 radiation was replaced in 1983 by one based on the speed of light. Also added were the prefixes zetta (Z) for 10, zepto (z) for 10 , yotta (Y) for 10 , and yocto (y) for 10 . 

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