Physics
Waves, frequency & sound — quick study summary
A wave transfers energy without transferring matter. Key properties: wavelength (λ), frequency (f), period (T = 1/f), amplitude, and speed (v = fλ). Transverse waves oscillate perpendicular to travel direction (light, water); longitudinal waves oscillate along it (sound, P-waves). Interference: constructive (in-phase) waves add, destructive (out-of-phase) cancel. The Doppler effect shifts frequency for a moving source — higher when approaching, lower when receding.
Key points
- v = fλ; T = 1/f
- Transverse: oscillates ⊥ to travel (light, water). Longitudinal: oscillates ∥ to travel (sound)
- Amplitude → intensity/loudness; frequency → pitch/colour
- Doppler: f_observed > f_source when approaching; less when receding
- Reflection: angle in = angle out. Refraction bends waves entering different media
Practice quiz
Click each question to reveal the answer.
1. Sound travels at ~340 m/s in air. What's the wavelength of a 1700 Hz sound?
- 0.2 m
- 2 m
- 20 m
- 0.02 m
Answer: 0.2 m
λ = v ÷ f = 340 ÷ 1700 = 0.2 m.
2. An ambulance siren sounds higher-pitched as it approaches and lower as it leaves. What's this called?
Answer: Doppler effect
Wavefronts compress in front of a moving source (higher frequency) and stretch behind it (lower).
3. What kind of wave is a sound wave?
Answer: Longitudinal — air molecules oscillate parallel to the direction of travel
Sound is a compression wave in a medium. Light, in contrast, is transverse (electric/magnetic fields oscillate perpendicular to travel).
Last reviewed: May 2026