Work Function & Threshold Frequency (OCR A Level Physics)

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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

Energy Well (1), downloadable AS & A Level Physics revision notes Energy Well (2), downloadable AS & A Level Physics revision notes Energy Well (3), downloadable AS & A Level Physics revision notes

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

Examiner Tip

A useful analogy for threshold frequency is a fairground coconut shy:

  • One person is throwing table tennis balls at the coconuts, and another person has a pistol
  • No matter how many of the table tennis balls are thrown at the coconut it will still stay firmly in place – this represents the low frequency quanta
  • However, a single shot from the pistol will knock off the coconut immediately – this represents the high frequency quanta

Coconut Shy Photoelectric Effect, downloadable AS & A Level Physics revision notes

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Katie M

Author: Katie M

Expertise: Physics

Katie has always been passionate about the sciences, and completed a degree in Astrophysics at Sheffield University. She decided that she wanted to inspire other young people, so moved to Bristol to complete a PGCE in Secondary Science. She particularly loves creating fun and absorbing materials to help students achieve their exam potential.