The photoelectric effect is the process by which photons of electromagnetic radiation shone onto a metal cause photoelectric emission of electrons from its surface. This is demonstrated by the gold leaf electroscope.
The top plate is charged by briefly touching it with a negative electrode from a high voltage power supply. Excess electrons are deposited on the plate, so the stem and gold leaf repel each other. This makes the leaf lift away from the stem. UV radiation is then shone on the zinc surface, and the gold leaf slowly falls back towards the stem, indicating that the electroscope has lost its negative charge due to photoelectric emission.
The electroscope raised some observations:
- Photoemission only occurred above a threshold frequency, below which even high intensity did not cause emission.
- Incident radiation above the threshold frequency caused instantaneous emission.
- Above the threshold frequency, increasing the intensity of radiation did not increase the maximum kinetic energy of electrons emitted. More electrons were emitted, but only frequency affected kinetic energy of the photoelectrons.
These observations could not be explained using the wave model of electromagnetic radiation, because greater intensity should increase energy imparted to the electrons and hence the maximum KE of electrons emitted. So a new model was needed.
Photons can explain the photoelectric effect, because their energy depends on their frequency, not their intensity. In this model, only one-to-one interactions between a photon and a surface electron can occur, so electrons cannot accumulate energy from multiple photons. This also explains the instantaneous nature of the emission. Einstein explained the third observation by introducing a constant called the work function: the minimum energy required to free an electron from the surface of the metal.
Above the threshold frequency, the rate of emission of photoelectrons is directly proportional to the intensity of the incident radiation. This is because greater intensity implies more electrons per second, and hence a greater rate of photoemission.
As a result of the principle of conservation of energy:
In other words, the energy of incident photons (ℎ) equals the energy needed for electrons to be released from the metal surface (φ) plus the KE that emitted electrons have.
The only way to increase the maximum kinetic energy of the emitted photoelectrons is to increase the frequency of radiation: increasing intensity may increase the rate of photoemission but will not increase the KE of photoelectrons.
The maximum kinetic energy of the photoelectrons is independent of the intensity of the incident radiation, but the rate of photoelectrons above the threshold frequency is directly proportional to the intensity of the incident radiation.
So, to summarise:
- Work function: is the minimum energy needed to remove a single electron from the surface of a particular metal.
- Threshold frequency: the minimum frequency of the electromagnetic radiation that will cause the emission of an electron from the surface of a particular metal, f.