What does the CCEA AS 2 Waves, Photons and Medical Physics unit cover?

AS 2 is the waves and modern-physics unit. It covers transverse and longitudinal waves, the wave equation, intensity and polarisation, superposition with interference, diffraction and stationary waves, refraction, the refractive index, total internal reflection, optical fibres and lenses, the photon model and the photoelectric effect with Einstein's equation, wave-particle duality with the de Broglie wavelength and electron diffraction, and medical physics covering X-rays, ultrasound and radioactive tracers.

Why does the photoelectric effect need the photon model?

The photoelectric effect shows a threshold frequency below which no electrons are emitted however bright the light, and emission that is instant. The wave model predicts that any frequency should eventually free electrons if the light is bright enough, and that there would be a time delay. Treating light as photons of energy hf, where one photon gives its energy to one electron, explains both the threshold and the instant emission, which is why the effect is direct evidence for photons.

What is the difference between a progressive wave and a stationary wave?

A progressive wave transfers energy from one place to another, and every point has the same amplitude. A stationary wave is formed when two identical waves travel in opposite directions and superpose; it has fixed nodes of zero amplitude and antinodes of maximum amplitude, and it stores energy rather than transferring it along the wave.

What is the most common mistake in AS 2?

In waves, students often forget that intensity is proportional to the square of the amplitude, or that only transverse waves can be polarised. In quantum physics, the frequent error is saying brighter light gives faster electrons; above the threshold, intensity increases the number of electrons but only a higher frequency increases their maximum kinetic energy.

How should I revise the AS 2 unit?

Work topic by topic against the specification statements. Learn the wave definitions and the wave equation, drill double-slit, diffraction grating and stationary-wave calculations, and practise Snell's law, critical angle and lens-equation problems. For quantum physics, rehearse Einstein's photoelectric equation and stopping-voltage graphs and the de Broglie wavelength. For medical physics, learn how X-rays, ultrasound and tracers work and why coupling gel and gamma emitters are used.

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CCEA A-Level Physics AS 2 Waves, Photons and Medical Physics: a complete overview of waves, the photoelectric effect and imaging

A deep-dive CCEA A-Level Physics guide to the AS 2 Waves, Photons and Medical Physics unit. Covers waves and their properties, superposition and stationary waves, refraction and lenses, quantum physics and the photoelectric effect, wave-particle duality, and medical physics, with the definitions and equations CCEA examines.

Generated by Claude Opus 4.818 min readCCEAUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

Jump to a section

What this unit demands
Waves and superposition
Refraction and lenses
Quantum physics and duality
Medical physics
How this unit is examined
Check your knowledge

What this unit demands

AS 2 Waves, Photons and Medical Physics moves from the classical physics of waves into the quantum description of light and matter, and applies both to medical imaging. The examiners test precise wave definitions, confident calculation with the wave, lens and photoelectric equations, and clear explanation of why the photon model is needed.

This guide walks through the six dot points of the unit, then sets out the exam patterns CCEA repeats. Each topic has a matching dot-point page with practice questions; this overview ties them together.

Waves and superposition

A transverse wave oscillates perpendicular to its travel and a longitudinal wave parallel to it. The wave equation $v = f\lambda$ links the quantities, intensity is proportional to amplitude squared, and only transverse waves can be polarised. The principle of superposition gives interference, with constructive interference at a path difference of $n\lambda$ , and stationary waves with nodes and antinodes form when identical waves travel in opposite directions.

Refraction and lenses

Refraction bends light at a boundary where its speed changes, governed by Snell's law and the refractive index $n = \frac{c}{v}$ . Beyond the critical angle light is totally internally reflected, the basis of optical fibres. A converging lens forms images found from the lens equation $\frac{1}{f} = \frac{1}{u} + \frac{1}{v}$ .

Quantum physics and duality

Light is delivered in photons of energy $E = hf$ . The photoelectric effect needs this model: below a threshold frequency no electrons are emitted however bright the light. Einstein's equation $hf = \phi + E_{k(\max)}$ conserves energy. Wave-particle duality is completed by electron diffraction and the de Broglie wavelength $\lambda = \frac{h}{p}$ , while discrete atomic energy levels explain line spectra.

Medical physics

X-rays are made by stopping fast electrons and pass through soft tissue but are absorbed by bone. Ultrasound reflects at boundaries of different acoustic impedance, with a coupling gel to remove the air gap. Radioactive tracers are gamma emitters with a short half-life that concentrate in a target organ and are detected outside the body.

How this unit is examined

A typical CCEA profile for AS 2:

Definitions. Wave types and quantities, polarisation, refractive index and the photon model.
Calculation. The wave equation, double-slit and grating problems, Snell's law and the lens equation, and Einstein's photoelectric equation.
Graphs. Stationary-wave patterns and the stopping-voltage against frequency line.
Application. Optical fibres, line spectra, and how X-rays, ultrasound and tracers are used in medicine.

Check your knowledge

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

State the difference between a transverse and a longitudinal wave. (2 marks)
A wave travels at $1500\,\text{m}\,\text{s}^{-1}$ with a frequency of $3.0\,\text{MHz}$ . Find its wavelength. (2 marks)
State the path-difference conditions for constructive and destructive interference. (2 marks)
Glass has a refractive index of $1.5$ . Find the critical angle for a glass-air boundary. (2 marks)
State Einstein's photoelectric equation and define each term. (3 marks)
Explain why no electrons are emitted below the threshold frequency. (2 marks)
Find the de Broglie wavelength of an electron of momentum $1.8 \times 10^{-24}\,\text{kg}\,\text{m}\,\text{s}^{-1}$ . Take $h = 6.6 \times 10^{-34}\,\text{J}\,\text{s}$ . (2 marks)
Explain why a coupling gel is used in ultrasound scanning. (2 marks)

Solutions

Q1: In a transverse wave the oscillations are perpendicular to the direction of energy transfer; in a longitudinal wave they are parallel to it.
Q2: $\lambda = \frac{v}{f} = \frac{1500}{3.0 \times 10^{6}} = 5.0 \times 10^{-4}\,\text{m}$ .
Q3: Constructive: path difference of a whole number of wavelengths, $n\lambda$ . Destructive: an odd number of half wavelengths, $(n + \tfrac{1}{2})\lambda$ .
Q4: $\sin\theta_c = \frac{1}{1.5}$ , so $\theta_c = 41.8^\circ$ .
Q5: $hf = \phi + E_{k(\max)}$ : $hf$ is the photon energy, $\phi$ the work function (minimum energy to release an electron) and $E_{k(\max)}$ the maximum kinetic energy of the emitted electron.
Q6: Each photon has energy below the work function, and one photon interacts with one electron, so no electron gains enough energy to escape, however bright the light.
Q7: $\lambda = \frac{h}{p} = \frac{6.6 \times 10^{-34}}{1.8 \times 10^{-24}} \approx 3.7 \times 10^{-10}\,\text{m}$ .
Q8: It removes the air gap between the transducer and the skin; without it the large acoustic impedance difference between air and skin would reflect almost all the ultrasound, so little would enter the body.

Sources & how we know this

CCEA GCE Physics specification — CCEA (2016)