What is a wave?
Drop a pebble into a pond. Ripples spread out, but the water itself mostly bobs up and down in place. A wave is a disturbance that carries energy from one place to another without carrying matter along.
Mechanical and electromagnetic waves
| Mechanical waves | Electromagnetic waves |
|---|---|
| Need a medium (solid, liquid or gas) | Don’t need a medium; travel through empty space |
| Sound, water waves, seismic waves, waves on a rope | Light, radio waves, X-rays, microwaves |
Transverse and longitudinal waves
| Transverse wave | Longitudinal wave | |
|---|---|---|
| Particle motion | Perpendicular to wave direction | Parallel to wave direction |
| Parts | Crests (high points) and troughs (low points) | Compressions (squished) and rarefactions (spread out) |
| Examples | Waves on a rope, light, ripples on water (mostly) | Sound, a slinky pushed and pulled |
Earthquakes produce both: P-waves (longitudinal, faster) and S-waves (transverse, slower).
Parts of a wave
- Amplitude: how far the particles move from rest. Larger amplitude = more energy (louder sound, brighter light, bigger ocean waves).
- Wavelength (λ, lambda): the distance from one crest to the next, or one compression to the next.
- Frequency (f): how many waves pass a point each second, in hertz (Hz). 1 Hz = 1 wave per second.
- Period (T): time for one wave to pass. T = 1 ÷ f.
The wave equation
wave speed = frequency × wavelength
v = f × λ
Example: A sound wave has a frequency of 440 Hz (the note A) and travels at 343 m/s. What is its wavelength?
λ = v ÷ f = 343 ÷ 440 = 0.78 m
For a given medium, speed stays the same. If frequency increases, wavelength decreases.
Sound waves
Sound is a longitudinal mechanical wave. Speed depends on the medium:
| Medium | Speed of sound |
|---|---|
| Air (20 °C) | 343 m/s |
| Water | about 1480 m/s |
| Steel | about 5900 m/s |
| Vacuum (space) | 0, sound can’t travel |
Pitch and frequency
Pitch is how high or low a sound seems. Higher frequency = higher pitch.
- Humans hear roughly 20 Hz to 20 000 Hz. The upper limit drops as people age.
- Infrasound (below 20 Hz): elephants and whales use it to communicate over long distances.
- Ultrasound (above 20 000 Hz): bats and dolphins use it; doctors use it to image babies and organs.
Loudness and amplitude
Loudness is measured in decibels (dB). Every 10 dB increase is 10 times more intense.
| Sound | dB |
|---|---|
| Rustling leaves | 20 |
| Conversation | 60 |
| Heavy traffic | 85 |
| Chainsaw | 110 |
| Jet engine nearby | 140 (pain threshold) |
Long exposure above 85 dB can cause permanent hearing loss.
Wave behaviours
| Behaviour | What happens | Example |
|---|---|---|
| Reflection | Bounces off a surface | Echoes, sonar |
| Refraction | Changes direction when speed changes | Sound bending over a cool lake at night |
| Diffraction | Spreads around obstacles or through openings | Hearing someone around a corner |
| Interference | Waves overlap and add or cancel | Noise-cancelling headphones |
| Absorption | Energy is taken in by a material | Soft furniture quieting a room |
The Doppler effect
When a sound source moves toward you, the waves bunch up, so the pitch sounds higher. When it moves away, the pitch sounds lower. You hear this with a passing ambulance siren. Police radar and weather radar use the Doppler effect to measure speed.
How we hear
- Sound waves enter the ear canal.
- They vibrate the eardrum.
- Three tiny bones (hammer, anvil, stirrup) amplify the vibrations.
- The cochlea, a fluid-filled spiral, turns vibrations into nerve signals.
- The brain interprets them as sound.
Common mistakes
- Thinking the wave carries the water or air along with it.
- Confusing amplitude and frequency. Amplitude is loudness; frequency is pitch.
- Thinking sound travels faster in air than in solids.
Try it yourself
- Stretch a slinky along the floor. Make a transverse wave and a longitudinal wave. Describe the difference.
- A wave has a frequency of 5 Hz and a wavelength of 2 m. What is its speed?
- A dog whistle sounds at 25 000 Hz. Why can’t you hear it?
- You see lightning and hear thunder 6 seconds later. About how far away was the strike?
(Answers: 10 m/s; it’s above the human range; 343 × 6 ≈ 2 km)
Quick recap
- Waves transfer energy, not matter.
- Transverse waves move perpendicular; longitudinal waves move parallel.
- Amplitude relates to energy; frequency relates to pitch.
- v = f × λ.
- Sound needs a medium and travels fastest in solids.