AQA GCSE · Trilogy 8464 · Physics Paper 2 · Spec section 6.6

P6 Waves

What properties do all waves share, and how does the electromagnetic spectrum connect visible light to radio waves and X-rays?

Waves transfer energy from place to place without transferring matter. The medium (if there is one) oscillates but doesn't travel with the wave. Transverse waves (light, EM radiation, water waves) oscillate perpendicular to the direction of travel; longitudinal waves (sound, seismic P-waves) oscillate parallel to it. Every wave has an amplitude, frequency, wavelength, and wave speed — related by the wave equation v = fλ.

The electromagnetic (EM) spectrum is a family of transverse waves, all travelling at 3 × 10⁸ m/s in a vacuum. They differ only in frequency and wavelength. The entire spectrum — from radio waves to gamma rays — carries different energy per photon (E = hf), which determines both their uses and their hazards. Infrared is heat radiation; ultraviolet causes sunburn; X-rays and gamma rays are ionising and damage DNA.

A hospital ultrasound scanner uses waves at 5 MHz. The speed of sound in tissue is approximately 1500 m/s. (a) Calculate the wavelength. (b) Explain why ultrasound is used rather than audible sound for medical imaging. (c) What property of a wave determines how much energy it carries?

Key equations

Wave equation: v = fλ recall

Period: T = 1/f equation sheet

What students must understand

Waves transfer energy, not matter; the medium oscillates in place
Transverse waves: oscillation perpendicular to direction of travel (EM, water waves, seismic S-waves)
Longitudinal waves: oscillation parallel to direction of travel (sound, seismic P-waves); compressions and rarefactions
Amplitude, wavelength, frequency, period: definitions and units
Wave equation: v = fλ (all waves, all contexts)
Reflection, refraction (wave changes speed and direction at a boundary), absorption, transmission
Sound: requires a medium; not transmitted through a vacuum; range of human hearing 20 Hz–20 kHz
Ultrasound (>20 kHz): medical scanning, sonar; echoes used to measure distance/depth
Seismic waves: P-waves (longitudinal) and S-waves (transverse); evidence for Earth's internal structure
EM spectrum (low to high frequency): radio → microwave → infrared → visible → UV → X-ray → gamma
All EM waves travel at c = 3 × 10⁸ m/s in a vacuum
Uses of EM radiation: radio (broadcasting), microwave (Wi-Fi, cooking), IR (thermal imaging, remote controls), visible (photography), UV (sterilisation, security marks), X-rays (medical imaging), gamma (cancer treatment, sterilisation)
Hazards: ionising radiation (UV, X-ray, gamma) damages DNA and can cause cancer
HT Reflection and refraction: angle of incidence, angle of reflection (from normal); Snell's Law at a qualitative level

Linking questions

How does gamma radiation from radioactive decay connect to this EM spectrum? → P4 Atomic Structure
How does wave superposition (interference) work at DP level? → DP Physics C.2–C.4 Waves