Waves, sound & light · Topic 11 of 17

Waves & Sound

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Physics · waves and sound

Waves carry energy, not matter. Transverse waves vibrate across their path, longitudinal waves along it; v = fλ for every wave; frequency sets pitch.

01 · Two typesTransverse ⟂ (light) · longitudinal ∥ (sound)

02 · Wave equationv = f × λ

03 · DopplerApproaching → higher pitch

A wave carries energy, not matter must know

λ = crest-to-crest distance; f = waves per second (Hz); amplitude = height from rest must know

Sound = compressions + rarefactions (a longitudinal pressure wave)

Standing wave: nodes (no motion) and antinodes (most motion)

Wave speed v = f × λ — for every wave, light included must know

Period T = 1 / f (seconds per wave)

Sound ≈ 343 m/s in air (20 °C); faster in liquids, fastest in most solids

Human hearing ≈ 20–20,000 Hz; above that = ultrasound

Pitch = perceived frequency: higher f → higher pitch must know

Bigger amplitude → louder (more energy), same pitch

Doppler: moving closer → higher pitch; moving apart → lower

v = 5 Hz × 3 m = 15 m/s

λ = 343 m/s ÷ 686 Hz = 0.5 m

Radio: λ = 3 × 10⁸ m/s ÷ 1 × 10⁸ Hz = 3 m

Transverse: vibration ⟂ travel (light); longitudinal: ∥ (sound) must know

Sound needs a medium; light can travel through a vacuum

Commonly confused

	Transverse	Longitudinal
Vibration vs travel	Perpendicular (at 90°)	Parallel (same line)
High/low points	Crests and troughs	Compressions and rarefactions
Examples	Light and other EM waves, waves on a string	Sound in air and water
Needs a medium?	Depends — light needs none; a wave on a string does	Yes — sound needs matter

Same medium: higher f → shorter λ (speed is set by the medium)

Show 3 more facts

Water waves mix transverse and longitudinal motion

Sound level in decibels (dB): ~10 dB faint, ~140 dB jet at 30 m

Warmer air → slightly faster sound

Pearl

A siren rising as it nears and falling as it leaves is the Doppler effect in one sound.

Practice this topic, then test the whole section.

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