The Photoelectric Effect (HL) (HL IB Physics)

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

• The photoelectric effect provides important evidence that light behaves as a particle i.e. it is quantised, or carried in discrete packets
  • This is shown by the fact each electron can absorb only a single photon

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

Threshold Frequency

• Photoelectrons are emitted from the surface of a metal when light of sufficient energy shines on it
• The frequency of the photons required for the photoelectric effect to occur is called the threshold frequency
• The threshold frequency of a metal is defined as:

The minimum frequency of incident electromagnetic radiation required to 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

Metal	Threshold Frequency f0 / Hz	Threshold Wavelength λ0 / nm
sodium	4.40 × 1014	682
potassium	5.56 × 1014	540
zinc	1.02 × 1015	294
iron	1.04 × 1015	289
copper	1.13 × 1015	266
gold	1.23 × 1015	244
silver	9.71 × 1015	30.9

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
  • One 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 the work function 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