This page covers the wave particle duality section of the course.
One way of modelling Newton’s corpuscular theory of light (Topic 2.1) is a rough demonstration of refraction using a ball rolling down a slope. Fold and support a large piece of card so that there are two flat surfaces separated by a short broad ramp. If a small ball is made to roll obliquely down the ramp from the higher surface to the lower, its direction changes (towards the normal) as it speeds up. The students can investigate Young’s fringes with either light or 3 cm microwaves, depending on the equipment you have available. An appropriate risk assessment should be carried out, especially if you are using a laser for the light source. You may be surprised to learn that Newton maintained his support for the corpuscular theory despite having studied Newton’s rings – which provide strong evidence in favour of the wave nature of light.
Electromagnetic waves (Topic 2.2) can be demonstrated using 3 cm microwaves. Most microwave kits include the extra equipment needed to demonstrate and investigate the properties of waves. For instance, students should be able to measure the wavelength by setting up a standing wave with an aluminium reflector and using the probe to locate the maxima and minima. Other wave properties that can be studied using 3 cm microwaves include diffraction and polarisation.
We can introduce wave–particle duality via the photoelectric effect (Topic 2.3), which shows why a particle theory of light won favour again. Students should understand the difference between photons and photoelectrons. Simply put, it is the photons that go in and photoelectrons that come out. The idea of a minimum energy may be hard to grasp, but a simple analogy for the photoelectric effect is for students to think of a sculptor chipping away at a stone block. Insufficient energy produces no result; a hard blow chips a little away; a harder blow can send a small chip flying. If you calculate some photon energies for different types of electromagnetic waves (using E = hf or E = hc/λ) they can relate this energy to the position in the electromagnetic spectrum, which they should recall from their GCSE work.
A simple way for understand how to measure the Planck constant is by using LEDs of different colours. Students should measure the forward bias voltage (V) that just switches each LED on. Manufacturers supply values for the wavelength (λ) of the light produced by their LEDs. Once students are clear that in this case the energy flow runs the other way from that in the photoelectric effect, they can substitute their values in eV = hc/λ.
The discovery of electron diffraction (Topic 2.4) substantiated de Broglie’s hypothesis. It is a useful exercise to calculate the de Broglie wavelength for various particles – for instance electrons moving in a TV tube or protons in an accelerator beam.
This chapter can be difficult. The concept of wave–particle duality involves new concepts. You should try to develop confidence with the formulae and calculations used to test understanding of this material in examinations.