Wave-Particle Duality
- Light waves can behave like particles, i.e. photons, and waves
- This phenomena 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
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 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 none of this is observed
- If the frequency is above the threshold and the intensity of the light is increased, more photoelectrons are emitted per second
- Although the wave theory provided good explanations for phenomena such as interference and diffraction, it failed to explain the photoelectric effect
Wave-Particle Duality: Electron Diffraction
- 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
- 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
Investigating Electron Diffraction
- Electron diffraction tubes can be used to investigate 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:
Ek = ½ mv2 = eV