Eduqas A-Level Physics Waves, photons and lasers: superposition, refraction, the photoelectric effect and duality

What this module actually demands

The waves and quantum content opens Component 3 (Light, Nuclei and Options). It builds from describing waves, through the wave phenomena of superposition, interference, diffraction and refraction, into the quantum behaviour of light revealed by the photoelectric effect, the operation of lasers, and the wave nature of matter. The examiners reward fluent wave calculations, precise quantum explanations, and clear ray and energy-level diagrams.

This guide walks through the topics in order and sets out the exam patterns Eduqas repeats. Each topic has a matching dot-point page with practice; this overview ties them together.

Waves, interference and refraction

The nature of waves distinguishes transverse from longitudinal waves, defines the wave quantities and the wave equation, links phase to path difference, and explains polarisation as a property of transverse waves only. Wave properties states the principle of superposition, analyses two-source interference and the Young double-slit experiment with $y = \frac{\lambda D}{a}$, uses the diffraction grating $d\sin\theta = n\lambda$, and describes stationary waves with nodes and antinodes.

Refraction of light defines the refractive index and applies Snell's law, explains the change of speed and wavelength, and covers total internal reflection, the critical angle and optical fibres.

Photons, lasers and duality

Photons describes the photon as a quantum of energy $E = hf$, explains the photoelectric effect and Einstein's equation, and defines the work function, threshold frequency and the electronvolt. Lasers covers discrete energy levels, spontaneous and stimulated emission, population inversion and the metastable state, and the properties of laser light. Wave-particle duality introduces the de Broglie wavelength $\lambda = \frac{h}{p}$ and electron diffraction as evidence that particles behave as waves.

How this module is examined

A typical Eduqas profile for this content:

• Calculations. Wave equation problems, double-slit fringe spacing, grating angles, Snell's law and critical angles, photon energy and photoelectric kinetic energy, energy-level transitions, and de Broglie wavelengths.
• Graph questions. Maximum kinetic energy against frequency for the photoelectric effect, and interpreting interference and diffraction patterns.
• Explanation and definition. Polarisation, coherence, the photoelectric threshold and the photon model, stimulated emission and population inversion, and electron diffraction as evidence for duality.
• Diagram work. Ray diagrams for refraction and optical fibres, and energy-level diagrams for emission.

Check your knowledge

A mix of recall and calculation questions covering the module. Attempt them under timed conditions, then check against the solutions.

1. State the difference between a transverse and a longitudinal wave. (2 marks)
2. A wave has frequency $40\ \text{Hz}$ and wavelength $0.85\ \text{m}$. Find its speed. (1 mark)
3. Light passes from air into glass of refractive index $1.5$. State what happens to its frequency. (1 mark)
4. State the equation for the energy of a photon. (1 mark)
5. State what is meant by stimulated emission. (2 marks)
6. An electron has momentum $5.0 \times 10^{-24}\ \text{kg m s}^{-1}$. Find its de Broglie wavelength ($h = 6.63 \times 10^{-34}\ \text{J s}$). (2 marks)