Introduction

It has been known since the nineteenth century that when light falls on certain metallic surfaces, electrons are sometimes released from the metal. This is known as the photoelectric effect, and the emitted particles are called photoelectrons. If a positively charged electrode is placed near the photoemissive cathode so as to attract the photoelectrons, an electric current can be made to flow in response to the incident light. The device thereby becomes a photoelectric detector of the optical field, and it has proved to be one of the most important of all photometric instruments. Various means exist for amplifying the photoelectric current. In one important device, known as the photomultiplier and shown schematically in Fig. 9.1, the photoelectrons are accelerated sufficiently that on striking the positive electrode they cause the release of several secondary electrons for each incident primary electron, and these electrons are then accelerated in turn to strike other secondary emitting surfaces. After 10 or more similar stages of amplification, the emission of each photoelectron from the cathode results in a pulse of millions of electrons at the anode, which is large enough to be registered by an electronic counter. By counting these photoelectric pulses we have an extremely sensitive detector of light.

It has been found experimentally that photoelectric emission from a given surface occurs only if the frequency of the incident light is high enough to exceed a certain threshold value (see Fig. 9.2).