A photomultiplier tube is a piece of equipment that converts incident light into an electrical signal. It does this by utilising the photoelectric effect, where photons striking a metal surface cause electrons to be emitted.
In a PMT, several “dynode” stages are used, each of which is a piece of metal. When the light hits the first dynode, electrons are emitted. A potential difference is then applied between each dynode, causing the electrons to accelerate and hit the next dynode with more force, releasing even more electrons.
A PMT can amplify a light signal by 100 million times!