[Kerry Wong] took apart a PM2L color analyzer (a piece of photography darkroom gear) and found a photomultiplier tube (PMT) inside. PMTs are excellent at detecting very small amounts of light, but they also have a very fast response time compared to other common detection methods. [Kerry] decided to use the tube to measure the speed of light.
There are several common methods to indirectly measure the speed of light by relating frequency to wavelength (for example, using microwave ovens and marshmallows). However, measuring it directly is difficult because of the scale involved. In only a microsecond, light travels almost 1000 feet (986 feet or 299.8 meters).
[Kerry’s] setup included a laser diode and an oscilloscope. He measured the time it took the laser to bounce off a mirror to develop a baseline time. Then he moved the mirror further away and repeated the measurement. By subtracting the baseline from the new measurement, constant delays in the test equipment cancel out.
The video below shows his results and also discusses more detail about the circuitry. We’ve seen people attempt this kind of measurement before with less success. If you’d rather walk before you run, you might think about measuring the speed of sound instead.