Electron Diffraction (OCR A Level Physics)

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

Experimental setup to demonstrate electron diffraction

• 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 = ½ mv² = eV

• Electrons are normally referred to as particles, however, diffraction is a wave-like behaviour

  • Therefore, electron diffraction provides evidence for the wave-like behaviour of particles

Diffraction of Electrons through Graphite

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

• He showed 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

Electrons accelerated through a high potential difference demonstrate wave-particle duality 

• In order to observe the diffraction of electrons, they must be focused through a gap similar to their size, 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, while a lower accelerating voltage increases the diameter of the rings