Waves
Types of waves
Transverse waves: oscillations are perpendicular (at right angles) to the direction of wave travel. Examples: all electromagnetic waves (light, radio, X-rays), water ripples, waves on a string.
Longitudinal waves: oscillations are parallel to the direction of wave travel. Examples: sound waves, ultrasound, P-waves (seismic primary waves).
A longitudinal wave consists of compressions (high pressure regions where particles are close) and rarefactions (low pressure regions where particles are spread out).
Key wave properties
- Amplitude A: maximum displacement from the equilibrium position (metres).
- Wavelength (λ): distance between two successive crests (or compressions) — the length of one complete wave cycle (metres).
- Frequency (f): number of complete wave cycles per second (hertz, Hz).
- Period (T): time for one complete oscillation. T = 1/f (seconds).
- Wave speed (v): distance the wave travels per second (m/s).
The wave equation: v = fλ (wave speed = frequency × wavelength).
All electromagnetic waves travel at c = 3 × 10⁸ m/s in a vacuum.
The electromagnetic spectrum
In order of increasing frequency (decreasing wavelength):
Radio → Microwaves → Infrared → Visible → Ultraviolet → X-rays → Gamma rays
Mnemonic: "R My It Vain Ux Group" (Radio, Micro, Infrared, Visible, UV, X-ray, Gamma)
| Region | Typical use |
|---|---|
| Radio | Broadcasting, communications |
| Microwave | Cooking, satellite communications, mobile phones |
| Infrared | Remote controls, thermal imaging, optical fibre communication |
| Visible | Sight, photography |
| UV | Sterilisation, detecting forged bank notes, sunbeds |
| X-rays | Medical imaging (bone fractures), airport security |
| Gamma | Cancer treatment (radiotherapy), sterilising food and medical equipment |
All EM waves: travel at 3 × 10⁸ m/s in vacuum, are transverse, transfer energy, can travel through a vacuum.
Reflection, refraction, and diffraction
Reflection: angle of incidence = angle of reflection (measured from normal).
Refraction: wave changes speed (and therefore direction, unless hitting a surface at 90°) when moving between materials of different density/optical density. Light slows in denser medium and bends towards the normal.
Diffraction: waves spread out after passing through a gap or around an obstacle. Significant when gap width ≈ wavelength. Long radio waves diffract around hills; light does not diffract noticeably through doors because wavelength ≪ gap size.
Sound
Sound is a longitudinal wave. In air, the speed of sound ≈ 340 m/s (much slower than light).
- Higher frequency → higher pitch.
- Greater amplitude → louder sound.
- Speed of sound is fastest in solids and slowest in gases (particles closer → faster vibration transfer).
Ultrasound (frequency > 20,000 Hz) is used in medical scanning (foetal imaging), sonar (depth measurement), and SONAR detection. The pulse–echo technique uses: distance = (v × t) / 2.
Hearing range: humans typically 20 Hz – 20,000 Hz.
⚠Common mistakes
- Calling sound a transverse wave — it is longitudinal.
- Forgetting to divide by 2 in pulse-echo — sound travels to the object AND back.
- Confusing frequency and amplitude — frequency affects pitch; amplitude affects loudness.
- Wrong EM order — gamma has highest frequency and shortest wavelength.
AI-generated · claude-opus-4-7 · v3-ccea-physics