Photon Doppler velocimetry

Photon Doppler Velocimetry is a way of measuring how fast things are moving. We can do this by using something called photons, which are tiny bits of light that can travel very quickly. To measure the speed at which something is moving, we send out a beam of photons and measure how quickly the photons come back to us. If the photons come back faster, then the thing is moving quickly, and if they come back slower then the thing is moving slowly.