Wave-Particle Duality (AQA A Level Physics): Revision Note

Wave-particle duality

• Light can behave as a particle (i.e. photons) and a wave

• This phenomenon is called the wave-particle nature of light or wave-particle duality

• Light interacts with matter, such as electrons, as a particle

  • The evidence for this is provided by the photoelectric effect

• Light propagates through space as a wave

  • The evidence for this comes from the diffraction and interference of light in Young's Double-Slit Experiment

  • Diffraction occurs when waves pass through a gap or around an obstacle and the waves spread out

Light as a particle

• Einstein proposed that light can be described as a quanta of energy that behave as particles, called photons

• The photon model of light explains that:

  • Electromagnetic waves carry energy in discrete packets called photons

  • The energy of the photons are quantised according to the equation E = hf

  • In the photoelectric effect, each electron can absorb only a single photon - this means only the frequencies of light above the threshold frequency will emit a photoelectron

• The wave theory of light does not support the idea of a threshold frequency

  • The wave theory suggests any frequency of light can give rise to photoelectric emission if the exposure time is long enough

  • This is because the wave theory suggests the energy absorbed by each electron will increase gradually with each wave

  • Furthermore, the kinetic energy of the emitted electrons should increase with radiation intensity

  • However, in the photoelectric effect, this is not what is observed

• If the frequency of the incident light is above the threshold and the intensity of the light is increased, more photoelectrons are emitted per second

• Although the wave theory provides good explanations for phenomena such as interference and diffraction, it fails to explain the photoelectric effect

Electron diffraction

• Louis de Broglie discovered that matter can behave as a wave

• Experiments show that a diffraction pattern is produced when a beam of electrons is directed at a thin graphite film

• Diffraction is a property of waves, and cannot be explained by describing electrons as particles

• In order to observe the diffraction of electrons, they must be focused through a gap similar in size to their de Broglie wavelength, such as an atomic lattice

• Graphite film is ideal for this purpose because of its crystalline structure

  • The gaps between neighbouring planes of the atoms in the crystals act as slits, allowing the electron waves to spread out and create a diffraction pattern

• The diffraction pattern is observed on the screen as a series of concentric rings

  • This phenomenon is similar to the diffraction pattern produced when light passes through a diffraction grating

  • If the electrons acted as particles, a pattern would not be observed, instead, the particles would be distributed uniformly across the screen

• It is observed that:

  • a larger accelerating voltage reduces the diameter of a given ring

  • a lower accelerating voltage increases the diameter of the rings

Investigating electron diffraction

• Electron diffraction tubes can be used to investigate the wave properties of electrons

• The electrons are accelerated in an electron gun to a high potential, such as 5000 V, and are then directed through a thin film of graphite

• The electrons diffract from the gaps between carbon atoms and produce a circular pattern on a fluorescent screen made from phosphor

• Increasing the voltage between the anode and the cathode causes the energy, and hence speed, of the electrons to increase

• The kinetic energy of the electrons is proportional to the voltage across the anode-cathode:

$E_k=\frac{1}{2}m{v}^2=eV$