Quantum physics

This is possibly the part of the A level course that is most satisfying to teach, mainly because the contents of this topic will be new to most students (depending on their GCSE exam board).

You could cover this topic through the history of the development of the theory of light - it is a really good chance to emphasise that science isn't simply about a series of geniuses who invent theories in isolation just by thinking about it. The story of quantum mechanics suggest collaboration and building upon others ideas. The question of whether light is a particle or a wave really starts with Isaac Newton whose treatise on optics described light as a stream of particles called “corpuscles”. This theory competed with Christiaan Huygens' theory of light travelling as a wave through a medium just like sound. Later Thomas Young demonstrated the wave behaviour of light through his double slit experiment.

This topic stops before developing further ideas of quantum mechanics including quantum entanglement. Some of the results of the wave-particle duality are very counterintuitive, but there is no need for students to develop ideas further than the observed phenomena and the implication for the scientific model of light and matter.

The topic covers:

  • describing the behaviour of photons and the interactions with and between subatomic particles.
    • it is important to ensure students understand the link with the Bohr model of the electron and that the energy that a photon carries has to correspond exactly with energy required to excite electrons through energy levels. Students who also study chemistry will realise the link between electron shells and energy levels. Energy must be converted from electron-volts into joules before used in the E=h×f formula. A common problem is that photon energies are often given in MeV and so the conversion factor of 1 eV = 1.6×10-19 J can be also recalled as 1 MeV = 1.6×10-13 J . In other words, it needs to be a million times bigger.
  • using the photon model to explain observable phenomena:
    • the Teaching Advanced Physics resources provide practical experiments for students to do. Measuring Plank's constant using the LEDs of different frequencies is an experiment that works well, so long as a tube is used to block out external light.
  • describe evidence supporting the photon model:
    • when developing ideas around the photoelectric effect, ensure the link is made to the idea that this was definitive evidence for the particle model.
  • wave-particle duality, particle diffraction:
    • students will be new to this idea, but explaining it in terms of its applications. For example, electron microscopes and doing a good demonstration of the effect should help them realise that despite the counterintuitive nature of the phenomenon, it is a real one.

Whilst this list provides a source of information and ideas for experimental work, it is important to note that recommendations can date very quickly. Do NOT follow suggestions which conflict with current advice from CLEAPSS or recent safety guides. eLibrary users are responsible for ensuring that any activity, including practical work, which they carry out is consistent with current regulations related to Health and Safety and that they carry an appropriate risk assessment. Further information is provided in our Health and Safety guidance.

Links and Resources

Episode 501: Spectra and Energy Levels

This resource contains examples of demonstrations that students can use. It is a good idea to use the diffraction grating to examine the spectra from different elemental sources, such as sodium lamps. 

There is a discussion of quantisation, and the idea that the energy levels correspond to specific frequencies of light. The energy levels in the hydrogen atom are detailed, and it is important for the students to be aquainted with these.

The experiments in this resource are all useful and complement the topic well.

publication year
2000 - 2009

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Episode 502: The Photoelectric Effect

This resource from the Institute of Physics contains questions for students to work through as well as the experiment described in the video below.

It also contains a further experiment to measure the threshold frequency by determining the stopping potential. The concept of stopping potential isn't required at A level, but this is a useful experiment to do. There is a further description of how Milikan used similar apparatus to determine the Plank constant.

publication year
2000 - 2009

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Photoelectric Effect

An excellent video that describes the gold leaf photoelectric effect experiment. This is a video for teachers to explain how to do the demonstration for your students and is clearly explained.

publication year
2010 to date

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Electron Diffraction Tube

A demonstration for teachers on how to safely setup and use the electron diffraction tube to show electrons behaving as a wave. If you do not have access to an electron diffraction tube, you could use this video to show students, but much better to do it in the lab.

publication year
2010 to date

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Episode 506: Particles as Waves

This resource contains worked calculations for students to carry out as well as a thorough explanation and history of the development of the de Broglie equation.

There are useful questions to use when discussing this development with students.

publication year
2000 - 2009

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