A photomultiplier tube is a device for measuring numbers of photons (for example, the flux of photons over the area of the aperture). A photon that enters the tube will be converted to an electrical signal which can be detected.
As a photon enters the tube, it will strike a photocathode. The photocathode emits an electron using the photoelectric effect. The electron emitted will be drawn to an electrode down farther along the photomultiplier tube that is held at a higher potential. (These electrodes are called dynodes in a photomultiplier.) As the electron is drawn towards the dynode, it will accelerate due to the electric field. The electron will gain enough energy to "knock out" other electrons in the target dynode. This process continues all the way down the photomultiplier tube, with the electrons being pulled by stronger and stronger successive dynode potentials. At the end of the tube, there will be a large electric signal, as all of the electrons strike a detector. Because one single photon causes such a large flux of electrons, the photomultiplier tube is an extremely sensitive instrument, which assists in increasing the efficiency of measurements.