A Little History

Come 1676 Ole Rømer and Jean Picard were timing the orbits of Jupiter's moon Io and noticed a dependency in the data that could only be accounted for if the speed of light was not infinite, as was the common belief at the time. Others who analysed the data came to a number around 300000 km/s, not bad.

In the mid 1800's two french physicists, who were not unknown to each other, created two experimental setups to more accurately determine the speed of light. Hippolyte Fizeau used a rotating disk with cut notches over an 8km distance to arrive at a speed of 313000 km/s (10% error) while 13 years later Léon Foucault arrived at a speed that was within 0.6% of the currently accepted value using a rotating mirror on a much smaller scale.

Both methods are detailed here with the later being employed to educate undergraduates on the subject.

More accurate measurements were performed by Michelson, Pease and Pearson in the 1930's inside a 1 mile long vacuum. Their apparatus was based on Léon Foucault's rotating mirror design and an arc lamp as a light source. The eventual number of 299'796 km/s was arrived at.

What is mind-blowing for me about all the above methods is that they were performed without the aid of lasers as a light source.

The WIKI page has a great write-up on the different modern methods and more of the people from history that I excluded here https://en.wikipedia.org/wiki/Speed_of_light

A third way to measure c, aside from the Time of Flight and Astronomical methods used in the above examples, can be found by the product of wavelength and frequency of an electromagnetic wave. Such methods include the accurate measurement of capacitance, standing waves inside metallic cavities, and interferometry.

Today the speed of light is a fixed international standard set at 299'792'458 m/s and is in turn used to define a metre. Luckily we can avoid the paradox of a second being the time it takes light to travel 1 metre given it is defined as "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom"

The fixing of c, I have read, was so scientists could spend more accurately measuring a metre. Probably because measuring time accurately is a far simpler proposition.

So the 532nm laser I have should have a frequency of 563THz if all is good and well with the world.