A level revision

The photoelectric effect

The photoelectric effect shows that electromagnetic waves can also act like particles which exchange energy of specific quantities. These packets of energy are called quanta or photons

The experiment

The photoelectric effect can be demonstrated using 'the gold leaf experiment':

• The gold leaf electroscope (right) is negatively charged using a voltage supply. This causes the gold leaf to repeal as both it and the metal rod are negatively charged.
• An ultraviolet light is shined upon the metal rod
• The gold leaf becomes limp and falls back towards the metal rod

For the gold leaf to be attracted back towards the metal rod charge needs to be lost, in this case electrons need to be removed. Somehow the light is causing the electrons to escape.

'Unexplained' observations

A number of observations of this experiment cannot be explained if light is behaving like a wave:

• Weak but high frequency light released electrons
• Light of less than a minimum frequency, depending on the metal in the rod, did not cause the loss of electrons. If light was a wave the energy would build up and an electron would eventually be released.
• Intensity of the light doesn't matter in terms of the energy given to the escaped electrons but frequency has an effect.

The explanation

Einstein proposed an explanation for the photoelectric effect. He suggested that electromagnetic radiation (such as ultraviolet light) deliver all their energy to a single electron inside the atoms of the metal. Metals are made up of 'free' or delocalized electrons and so the energy of an incoming photon can be used to free an electron from the metal, any left over energy from the photon is transferred to kinetic energy of the electron.

The amount of energy each photon has is related to its frequency by the relationship:

Where h is Plank's constant which has a value of around 6.63 x 10^-34Js

More in depth

As mentioned earlier if an incoming photon does not have a high enough frequency for it to have sufficient energy to release an electron, nothing will happen. The minimum amount of energy needed is called the Work function and is denoted by the Greek letter this value is different for all metals.

As photons may have more than the minimum energy and this is transferred as kinetic energy the full equation relating to the photoelectric effect is:

This equation says that the maximum kinetic energy of an escape electron is equal to its energy (which is equal to hf, as above) minus the energy used to release it from the metal.