Sam Han of Queenstown asks :-

How do scientists measure the speed of light?

William Tobin, an astronomer at the University of Canterbury and author of the book "The Life and Science of Léon Foucault", responded.

They don't any more: they determine the length of the metre. Let me explain.

The ancients believed light travels instantaneously, but evidence of its finite speed came in 1676 when the Danish astronomer Ole Romer studied Jupiter's moons. As they orbited, the moons' regular eclipses behind Jupiter's disc were sometimes early and sometimes late depending on whether the Earth was closer or further from Jupiter. Romer understood that light took different times to cover different distances.

For the next two centuries the approximately 300,000-kilometre-per-second speed of light was determined by dividing the known diameter of the Earth's orbit by Romer-type estimates of the light-time to cross it. But in 1862 a French physicist, Léon Foucault, made the first accurate laboratory measurement, using the displaced reflection in a fast-spinning mirror to time light pulses over a 40-metre path. Spinning mirrors were used until the 1930s, but were displaced by more sophisticated techniques that bounced light in cavities or used radar and radio.

Multiplying a wave's frequency and wavelength yields its speed, and latterly the speed of light was determined from accurate frequency and wavelength measurements of special radiations, such as red laser light. By 1983 it was possible to measure the wavelengths of highly-stable lasers with a precision exceeding the then non-laser standard of length. The metre was redefined as the distance travelled by light in a vacuum in 1/299,792,458 of a second. Light's speed became an exact, defined quantity, of 299,792,458 metres per second, leaving the metre to be determined by experiment, which laser scientists in standards laboratories strive to do with ever-improved accuracy to meet the needs of science and industry.