Work Function & Threshold Frequency (OCR AS Physics): Revision Note

Work Function & Threshold Frequency

• The photoelectric effect provides important evidence that light is quantised or carried in discrete packets

  • This is shown by the fact each electron can absorb only a single photon

  • This means only the frequencies of light above a threshold frequency will emit a photoelectron

Threshold Frequency & Wavelength

• The threshold frequency is defined as:

  The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal

• The threshold wavelength, related to threshold frequency by the wave equation, is defined as:

  The longest wavelength of incident electromagnetic radiation that would remove a photoelectron from the surface of a metal

• Threshold frequency and wavelength are properties of a material, and vary from metal to metal

Threshold frequencies and wavelengths for different metals

Work Function

• The work function Φ, or threshold energy, of a material, is defined as:

  The minimum energy required to release a photoelectron from the surface of a metal

• Consider the electrons in a metal as trapped inside an ‘energy well’ where the energy between the surface and the top of the well is equal to the work function Φ

• A single electron absorbs one photon

• Therefore, an electron can only escape from the surface of the metal if it absorbs a photon which has an energy equal to Φ or higher

In the photoelectric effect, a single photon may cause a surface electron to be released if it has sufficient energy

• Different metals have different threshold frequencies and hence different work functions

• Using the well analogy:

  • A more tightly bound electron requires more energy to reach the top of the well

  • A less tightly bound electron requires less energy to reach the top of the well

• Alkali metals, such as sodium and potassium, have threshold frequencies in the visible light region

  • This is because the attractive forces between the surface electrons and positive metal ions are relatively weak

• Transition metals, such as zinc and iron, have threshold frequencies in the ultraviolet region

  • This is because the attractive forces between the surface electrons and positive metal ions are much stronger