The Photoelectric Effect

The Photoelectric Effect is the second lecture within the Quantum Phenomena section of PH1012. It covers the experiment behind and use of the photoelectric effect.

Previous: Introduction to Quantum Physics

Next: Particle Theory of Light

Experiment


The photoelectric effect describes incident light causing electrons to be emitted from a metal target. It only occurs above specific wavelengths of light - but this threshold frequency differs depending on the metal used. The battery is used to create a variable potential difference between the plates (the retarding voltage) which eventually reaches a stopping voltage beyond which current cannot flow through the circuit. The stopping voltge relates to the maximum kinetic energy of the ejected electrons by Eundefined.

Maths
The threshold frequency is typically ~5x1014Hz, and is dependent on the metal but NOT on the intensity of the incident light. This disagrees with the wave theory of light, which states that energy ∝ intensity ∝ kinetic energy of electrons. However, when light is considered as a stream of particles energy conversion states that hv = Ek + W + Ecollisions. hv is the energy of a photon, W is the metal's work function (an inherent property), and Ecollisions is the energy that the electrons lose due to internal colisions before they exit the metal. Electrons with maximum kinetic energy are those who have fewest internal collisions.

This can be expressed as Vo = hv/e - W/e, which provides a straight line graph.

Summary
Electrons can be forced to exit a metal when struck by incident light of high energy. Their release occurs independent of incident intensity, and their energy is determined by the energy of the incidnt photon, the number of internal collisions and the metal's workfunction.