How does a photomultiplier tube detect extremely weak light?

A photomultiplier tube detects extremely weak light by leveraging the photoelectric effect and a process of massive electron amplification.12

When light strikes the tube's photosensitive cathode, it causes the emission of electrons. These initial electrons are accelerated toward a series of additional electrodes known as dynodes, each held at a successively higher positive potential.12

The detection process relies on secondary emission: when an electron strikes a dynode, it triggers the release of several additional electrons. This cascade effect repeats at each of the multiple dynode stages, resulting in an exponential increase in the number of electrons. By the time the signal reaches the final dynode, the original photocurrent has been amplified by a factor of up to 1,000,000, allowing the device to measure low light intensities or even detect a single photon.12