When light falls upon the window, electrons are emitted from the cathode and are focussed upon the first dynode, which has a positive voltage to attract the negative electrons.

When an electron hits this first dynode it causes several electrons to be emitted from it.

This is called SECONDARY EMISSION.

These are attracted by the second dynode which cause a further multiplication of electrons.

By the time the electrons have reached the final anode the current is very much larger than the original cathode current.

The photo-multiplier is used for detecting extremely low values of light.