The Speed of LightWe cannot live only for ourselves. A thousand fibers connect us with our fellow men. - Herman Melville
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Speed of Light

Armand Fizeau was the first person to successfully measure the speed of light.

One of the ideas that Rene Descartes (1596 - 1650), the early Renaissance scientist and first person to suggest that different colors were associated with different periodic motions, got it wrong regarding the speed of light. Descartes had thought that light, as a disturbance in ether, traveled instantaneously.

Galileo unsuccessfully tried to measure the speed of light by having two men miles apart send light signals back and forth at night. The speed of light is far too fast to be measured this way.

The Danish astronomer, Olaf Roemer (1644 - 1710) was able measure the speed of light by measuring the variations in the timing of eclipses of Jupiter's moons. Specifically, he measured a 1000 second variation in the timing over the course of a year. Roemer then correctly concluded that it took the light 1000 seconds to travel across the diameter of the earth's path around the sun, or twice the distance to the sun. This technique and the results obtained by using it were essentially accurate.

In 1849, Armand Fizeau (1819 - 1896) measured the speed of light on earth by shining a beam of light at a mirror 8 km away through the teeth of a gear. At low speeds, an observer could see the reflected light between the gear teeth, but when the gear was rotated fast enough, the reflected light beam became obscured by the next tooth. The speed of light was than calculated from the speed of the rotating gear.

Speed of Light in Water

In 1850, the French scientist Jean Bernard Foucault (1819 - 1868) succeeded in measuring the speed of light in water and found it significantly reduced from the speed of light in air.

This difference in the speed of light in different materials creates refraction which we'll talk about in the next module.

 

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