Measuring the Speed of Light

How was the speed of light measured?

In this page, we will collect information on how the speed of light was originally measured. Try to carry out the computations for these various methods.

Overview (via John Baez)

We list four determinations of the speed of light. The first one by Rømer established that the speed of light was finite and associated a number to it. Fizeau's measurement is the first terrestrial measurement (in the sense that it did not use extra-terrestrial objects such as satellites of Jupiter etc.).

Rømer's 1676 measurement

Bradley's 1726 measurment

Fizeau's 1849 measurement

Foucault's 1862 measurement

The SI unit for length

Since 1983, the meter is defined as the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

Reference: Definition of Meter in SI units at NIST

  • Do you understand the reason for the weird denominator in the current definition of the speed of light in vacuum?
  • Do you understand why the unit of length was changed in 1983?

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