At what speed light travels? How do scientists measure the exact speed?

Ole Rømer

Light travels at the speed of 2,99,792 kilometers per second. We know the exact figure because it has been measured in various ways, and all the measurements agree. It was Galileo who made the first attempt to measure it. He had two observers some miles apart on a clear night, and gave each a lantern which could be covered by a screen. One observer was to uncover his lantern, and the other, as soon as he saw the first observer’s light, was to uncover his lantern. The period between the uncovering of both the lanterns seen was to be measured. Galileo found, however, that light traveled so rapidly that the distance between the two observers was too small for its speed to be measured. Danish astronomer Ole Roemer (picture above), working in Paris, made the first real estimate of the speed of light in 1676, and came to the conclusion that it was 3,09,000 kilometers a second, a figure  later proved to be, remarkably enough, approximately correct. He noticed, in observing the eclipses of the moons of Jupiter, that the intervals between the eclipses of one moon were not always the same, being 16 minutes, 26 seconds greater at one time of the year than at another. He decided that this difference could not be due to any real difference in the period of the eclipses, but must be caused by the greater or lesser distance over which the light had to travel from Jupiter to the Earth when the Earth was at different parts of its orbit, and therefore nearer to or farther from Jupiter.

Different methods of measuring the speed of light have been adopted since, and they all give practically the same figure. Two great French scientists, Hippolyte Fizeau and Leon Foucault, about the middle of the 19th century did the pioneer work. They flashed beams of light back and forth between systems of mirrors and lenses. Fizeau interrupted the beam by the teeth of the revolving cog-wheel; Foucault made one of the mirrors revolve. In each case the speed of light was calculated from the speed of the cog-wheel or of the mirror, but the actual method is too complicated to be described.

Albert Michelson

Both these scientists measured the speed of light to be 2,98,000 kilometers per second. The greatest modern measure of the speed of light was Albert Michelson (photo, left). Nearly all his life Michelson worked on his great problem. While still a young officer serving as instructor of physics at the Annapolis Naval Academy, he greatly improved Foucault’s method and obtained much more accurate results in 1878. In 1924 Michelson resumed his work on the speed of light. He flashed light back and forth between various California mountain peaks, over a distance of as much as 140 kilometers. In 1926 he announced the speed of light as 3,00,000 kilometers a second. Modern scientific experiments indicate that the speed of light is about 2,99,792.45 kilometers per second.

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