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Georges Sagnac (1869-1928) was a French physicist who lent his name to the Sagnac effect, a phenomenon which is at the basis of interferometers and ring laser gyroscopes developed since the 1970s.

Sagnac entered the École Normale Supérieure in 1889. While still a lab assistant at the Sorbonne, he was one of the first people in France to study X-rays, following Wilhelm Conrad Röntgen. He belonged to a group of friends and scientists that notably included Pierre and Marie Curie, Paul Langevin, Jean Perrin, and the mathematician Émile Borel. Marie Curie says that she and her husband had traded ideas with Sagnac around the time of the discovery of radioactivity.

In 1913, Georges Sagnac showed that if a beam of light is split and sent in two opposite directions around a closed path on a revolving platform, and then the beams are recombined, they will exhibit interference effects. From this result Sagnac concluded that light propagates at a speed independent of the speed of the source. The effect had been observed earlier (by Harress in 1911), but Sagnac was the first to correctly identify the cause.

The Sagnac effect (in vacuum) is consistent with stationary ether theories (such as the Lorentz ether theory) as well as with Einstein's theory of relativity. It is generally taken to be inconsisent with emission theories of light, according to which the speed of light depends on the speed of the source.

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