An undergraduate laboratory experiment to accurately measure the speed of light

In 1983 the speed of light was set as an exact quantity. It is now one of the fundamental constants in physics, with the meter being directly related to this speed and the definition of the second. As such, experiments that calculate the speed of light with high precision are important in university undergraduate laboratories. In the experiment discussed here, a method is described that allows the speed of light to be calculated using an apparatus that fits onto a 1m bench-top. An advantage of this method is that a simple digital voltmeter can be used. This measures the amplitude-modulated output from a laser beam emerging from a multi-pass cell that is mixed with that from a reference
beam. The relative phase shift is determined as the modulation frequency changes, allowing the speed of light to be ascertained. Full details of the apparatus and the electronics designed for the experiment are presented.