• The?photoelectric effect?is the phenomena in which electrons are emitted from the surface of a metal?upon the absorption of electromagnetic radiation
• Electrons removed from a metal in this manner are known as?photoelectrons
• 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

Photoelectrons are emitted from the surface of metal when light shines onto it

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

• The frequency and wavelength are related by the equation

• Since photons are particles of light,?v?=?c?(speed of light)

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

Threshold frequencies and wavelengths for different metals

The 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