P6.1 Waves in Air, Fluids and Solids
Transverse and longitudinal waves
Transverse waves: Oscillations are perpendicular to the direction of energy transfer.
- Examples: light, all electromagnetic waves, water surface waves, seismic S-waves.
Longitudinal waves: Oscillations are parallel to the direction of energy transfer. Contain compressions (regions of high pressure) and rarefactions (low pressure).
- Examples: sound, seismic P-waves, ultrasound.
Memory aid: Trans = across; the vibration crosses the direction of travel.
Key wave quantities
| Quantity | Symbol | Unit | Definition |
|---|---|---|---|
| Amplitude | A | m | Maximum displacement from equilibrium |
| Wavelength | λ | m | Distance of one complete wave cycle |
| Frequency | f | Hz | Number of complete waves per second |
| Period | T | s | Time for one complete wave cycle |
| Wave speed | v | m/s | Speed at which the wave pattern moves |
Period and frequency:
T = 1/f (or f = 1/T)
Wave speed equation
v = fλ
v = wave speed (m/s), f = frequency (Hz), λ = wavelength (m)
Worked example: Sound has a frequency of 440 Hz and a wavelength of 0.77 m.
v = 440 × 0.77 = 338.8 m/s ≈ 340 m/s ✓ (speed of sound in air)
Worked example: Light travels at 3 × 10⁸ m/s with wavelength 600 nm (600 × 10⁻⁹ m).
f = v/λ = (3 × 10⁸) / (600 × 10⁻⁹) = 5 × 10¹⁴ Hz
Properties of waves
Reflection: Wave bounces off a surface (angle of incidence = angle of reflection).
Refraction: Wave changes speed (and direction, unless hitting surface at 90°) when entering a different medium.
- Sound travels faster in solids than liquids than gases.
- Light travels slower in denser media.
Sound in different media:
- Sound requires a material medium — cannot travel through a vacuum.
- Speed of sound: ~340 m/s in air; ~1,500 m/s in water; ~5,000 m/s in steel.
Wave representation
Transverse wave diagram: sinusoidal curve; amplitude = maximum height above centre; wavelength = distance between two peaks.
Longitudinal wave diagram: series of vertical lines — close together (compression) and spaced apart (rarefaction).
Required practical — measuring wave speed
Ripple tank: Measure frequency using stroboscope; measure wavelength from image of wave pattern; calculate v = fλ.
Slinky spring: Generate transverse or longitudinal waves; measure wavelength and time one wave to get speed.
Common exam errors
- Confusing transverse and longitudinal — sound is longitudinal, light is transverse.
- Forgetting that wavelength is in metres when using v = fλ.
- Saying waves transfer matter — waves transfer energy, not matter (particles oscillate about fixed positions).
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