The Speed of Light

Light propagates through space as a wave most of the time and so exhibits wave-like behaviors, including the fact that it has a characteristic speed in any medium. The speed of light is large but finite and has a value close to 3 x 10^8 m/s (which translates to 186,000 mi/s). It is customary to measure the speed of an object by marking a known distance and measuring the time needed to travel that distance. Galileo had tried to measure the speed of light this way: He stood on a hill holding a covered lantern A. He uncovered the lantern so that light from it traveled to a distant hill where an associate held lantern B. As soon as the associate saw the light from A, he was to uncover the light at B so that the beam could make the return trip. Galileo attempted to measure the transit time but could not because he had no timepiece that could measure the small amount of elapsed time involved. (Mechanical clocks, such as the marvelous ships' clocks invented by Thomas Harrison, were still some fifty years away.)
If v = d/t and v is some big number like 3 x 10^8, it should come as no surprise that either the distance used must be very large or the time must be very small. Thus, the first successful attempt to measure the speed of light was conducted in 1676 by Ole Roemer, a Danish astronomer. The known distance he used was the distance from Jupiter to the Earth. He relied on the four large satellites of Jupiter and measured their periods (time for one orbit about the planet). Roemer noted that the satellites behaved like clockwork (pun intended: after all, events in the heavens are a clockwork defining days months and years. He was able to measure the time between eclipses of a given satellite and noted that the time grew as the the planets moved further apart.

The first terrestrial measurement of the speed of light was made in 1849 by Hippolyte Fizeau. His genius provided him a way to use a distance of a few kilometers and then determine the travel time for bursts of light. To do this he shined a beam of light at a rotating toothed wheel. We are talking 500 teeth in the wheel rotating at something like 30 rev/s. Whenever a gap in the wheel was in place, a pulse left the apparatus and traveled to a distant mirror (probably to a distant cube-corner--an arrangement of three mutually perpendicular mirrors that receive any incoming beam and return it to its source along a path parallel to the incoming pulses). The returning pulse could be seen only if the toothed wheel had moved enough (read: one gap) to let the light through. So by carefully measuring the rate of rotation, he effectively measured the travel time for the light pulses.

In this country, the best known attempt at measuring c was done by Albert A. Michelson. He replaced Fizeau's toothed wheel with an eight-sided rotating mirror.

A concise but thorough review of this topic can be found at


   and check out this topic.


A brief history of c, the speed of light

Return to Ray optics

This page was last reviewed by mgosselin 10/09/2005