The Experimental Methods to Determine The Speed of Light

Nowadays, the speed of light is a clear example of how accurately values can be measured, it is so firmly established that even the meter is defined in terms of it. However, before the 17th century nobody could approximate the speed of light, and many scientists thought that it is infinite, and that light could travel any distance instantaneously.

Galileo Galilei was one of the first to try to measure the speed of light. Yet, his method was too simple and could not provide any conclusive results, which led him to the idea that light travels at least ten times faster than sound.

Next, during the 17th and 18th centuries, there were two significant measurements of the speed of light. One of them was the experiment carried out by a Danish astronomer Ole Roemer, who measured the speed of light to be 240,000 kilometres per second by finding the time that light travelled from the moons of Jupiter when the Earth was closer and farther away from Jupiter.

Whereas James Bradley, an English physicist, used stellar aberration to find the speed of light in a vacuum, which gave him a value of 301,000 kilometres per second, which is quite close to the accepted value.

A century later, a very famous experiment was carried out by a french scientist named Armand Fizeau, who used a cog wheel and a mirror. These were placed eight kilometres apart, and Fizeau observed that when the cog wheel was rotated fast enough, the light actually struck one of the cogs when travelling back from the mirror. He knew how fast the cog wheel rotated, and measured the time for the wheel to move a width of a single cog, which he thought would be equal to the time that light travelled to the mirror and back. The result he obtained was 313,300 kilometres per second. Yet, this again was not accurate enough.

In 1862, a very close value was obtained by Jean-Bernard-Leon Foucault. Foucault improved Fizeau’s apparatus by replacing the cogwheel with a rotating mirror, which led to the apparatus being named as Fizeau-Foucault Apparatus. Foucault obtained a value of 299,796 kilometres per second for the speed of light. Whereas the speed of light is defined to be 299,792.458 kilometres per second according to a 1983 declaration by the 17th General Congress on Weights and Measures. Later on, experimental methods became increasingly accurate due to the development of new and highly accurate technology.