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Light Speed Measurement

August 4th Fact-of-the-Day

Light and radio waves are both electromagnetic radiation. The difference between them is merely frequency (or wavelength, which expresses the same characteristic differently). Because they are the same kind of energy, they propagate through a vacuum at the same speed. That speed is known to be very close to 2.997925 x 10^8 meters per second (186,282 miles per second). However, precise speed measurement is illusive. The most common measurement method is to measure the time required for a narrow pulse of radiation to pass two points a known distance apart. That seems simple enough, but creating a pulse is equivalent to an interference among a series of adjacent frequencies (explained in J.B. Fourier's 1822 book 'Théorie analytique de la chaleur'). Any pulse of electromagnetic energy therefore consists of a group of adjacent frequencies that produce sum-product interferences with each other. It can be shown by simple addition of sine oscillations that if the component frequencies have different velocities the group has a different velocity than the waves comprising it. ©2005 Martek, International. All rights reserved.

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