How are light and matter similar?

In this area of study students explore the design of major experiments that have led to the development of theories to describe the most fundamental aspects of the physical world – light and matter.
When light and matter are probed they appear to have remarkable similarities. Light, which was previously described as an electromagnetic wave, appears to exhibit both wave-like and particle-like properties. Findings that electrons behave in a wave-like manner challenged thinking about the relationship between light and matter, where matter had been modelled previously as being made up of particles.

On completion of this unit the student should be able to provide evidence for the nature of light and matter, and analyse the data from experiments that supports this evidence.

Key knowledge

Behaviour of light

  • investigate and describe theoretically and practically the effects of varying the width of a gap or diameter of an obstacle on the diffraction pattern produced by light and apply this to limitations of imaging using light
  • analyse the photoelectric effect with reference to:
    – evidence for the particle-like nature of light
    – experimental data in the form of graphs of photocurrent versus electrode potential, and of kinetic energy of electrons versus frequency
    – kinetic energy of emitted photoelectrons: E_{kmax} = hf-\phi, using energy units of joule and electron-volt
    – effects of intensity of incident irradiation on the emission of photoelectrons
  • describe the limitation of the wave model of light in explaining experimental results related to the photoelectric effect.

Matter as particles or waves

  • interpret electron diffraction patterns as evidence for the wave-like nature of matter
  • distinguish between the diffraction patterns produced by photons and electrons
  • calculate the de Broglie wavelength of matter: \lambda=\frac{h}{p}

Similarities between light and matter

  • compare the momentum of photons and of matter of the same wavelength including calculations using: p=\frac{h}{\lambda}
  • explain the production of atomic absorption and emission line spectra, including those from metal vapour lamps
  • interpret spectra and calculate the energy of absorbed or emitted photons: \Delta E=hf
  • analyse the absorption of photons by atoms, with reference to:
    – the change in energy levels of the atom due to electrons changing state
    – the frequency and wavelength of emitted photons:E=hf=\frac{hc}{\lambda}
  • describe the quantised states of the atom with reference to electrons forming standing waves, and explain this as evidence for the dual nature of matter
  • interpret the single photon/electron double slit experiment as evidence for the dual nature of light/matter
  • explain how diffraction from a single slit experiment can be used to illustrate Heisenberg’s uncertainty principle
  • explain why classical laws of physics are not appropriate to model motion at very small scales.

Production of light from matter

  • compare the production of light in lasers, synchrotrons, LEDs and incandescent lights.


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