(a) Explain what is meant by photoelectric emission (b) Draw a labelled diagram showing the structure of a simple type of a photocell and explain its mode o...

(a) Photoelectric emission

Photoelectric emission is the emission of electrons from the surface of a metal when electromagnetic radiation of sufficiently high frequency falls on it.

(b) Simple photocell

The photocell consists of an evacuated glass or quartz bulb containing a curved photosensitive cathode, usually coated with caesium, and a collecting anode. When light of frequency equal to or greater than the threshold frequency falls on the caesium-coated cathode, photoelectrons are emitted. The positively charged anode attracts and collects these electrons. Electrons then flow through the external circuit and a current is registered by the microammeter. Increasing the intensity of light increases the number of electrons emitted per second and hence increases the photocurrent.

(c) Applications of photoelectric emission

1. Automatic switching on of street lights at dusk.
2. Burglar and security alarm systems.
3. Television camera tubes.
4. Industrial counting and control devices, for example counting objects moving on a conveyor belt.

(d)

Before photoelectric emission begins, the light energy is absorbed by electrons in the metal. For light below the threshold frequency, the energy supplied to each electron is less than the work function, so the electron cannot escape from the metal surface; the absorbed energy is dissipated mainly as heat. At the threshold frequency, each photon supplies just enough energy to overcome the binding energy of an electron, and emission begins.

One factor affecting the number of emitted electrons is the intensity of the incident light, provided that its frequency is at least the threshold frequency. Greater intensity produces more emitted electrons per second.