What is a wave?
At a stadium, fans stand up and sit down one after another, creating a "wave" that ripples around the stands. Nobody runs around the stadium, yet the wave travels all the way around. Physical waves work the same way.
A wave is a disturbance that transfers energy from one place to another without carrying matter along with it.
Waves are everywhere:
- ripples on a pond
- sound from a speaker
- light from the Sun
- radio signals to your phone
- earthquakes shaking the ground
- microwaves heating your food
Waves carry energy, not matter
A rubber duck on a pond bobs up and down as waves pass, but it doesn’t travel across the pond. The water moves in small circles, while the energy moves forward. In the ocean, waves can carry energy thousands of kilometres from a distant storm.
Two families of waves
| Mechanical wave | Electromagnetic wave |
|---|---|
| Needs a medium (solid, liquid or gas) to travel through | Can travel through empty space (a vacuum) |
| Sound, water waves, earthquake waves, waves on a rope | Light, radio waves, microwaves, X-rays |
| Can’t travel in space | Travels at about 300 000 km/s in a vacuum |
That’s why astronauts can see the Sun’s light but can’t hear explosions on it.
Two ways waves move
| Transverse wave | Longitudinal wave |
|---|---|
| Medium moves at right angles (up and down) to the wave’s direction | Medium moves back and forth along the wave’s direction |
| Has a crest (high point) and a trough (low point) | Has compressions (squeezed) and rarefactions (spread out) |
| A wave on a rope, light, ripples on water | Sound, a pushed slinky |
You can see both with a slinky:
- Shake one end side to side → transverse wave.
- Push and pull one end → longitudinal wave.
Earthquakes produce both: fast longitudinal P-waves arrive first, then slower transverse S-waves.
Parts of a wave
| Part | Meaning |
|---|---|
| Crest | Highest point of a transverse wave |
| Trough | Lowest point |
| Wavelength | Distance from one crest to the next (or one compression to the next) |
| Amplitude | Height from the middle (rest) line to a crest; bigger amplitude = more energy |
| Frequency | Number of waves passing a point each second, in hertz (Hz) |
| Period | Time for one complete wave |
Wave speed
wave speed = frequency × wavelength
v = f × λ (λ is the Greek letter lambda)
Example: Ocean waves arrive every 5 seconds (frequency 0.2 Hz) with a wavelength of 40 m.
Speed = 0.2 × 40 = 8 m/s
Waves in daily life
| Wave | Used for |
|---|---|
| Sound | Talking, music, ultrasound images |
| Light | Seeing, photography, fibre-optic internet |
| Radio | Radio, TV, cell phones, Wi-Fi |
| Microwaves | Cooking, radar |
| Infrared | TV remotes, thermal cameras |
| X-rays | Medical imaging |
| Seismic waves | Studying Earth’s interior |
| Ocean waves | Surfing; wave energy for electricity |
Common mistakes
- Thinking waves carry the water or air forward. They carry energy.
- Thinking sound can travel through space.
- Mixing up wavelength and amplitude. Wavelength is length along the wave; amplitude is height.
Try it yourself
- Lay a rope on the floor with a friend holding the other end. Make transverse waves. Try making the waves faster. What happens to the wavelength?
- Stretch a slinky and make both transverse and longitudinal waves. Draw each.
- Float a cork in a tub of water and make ripples. Watch how the cork moves.
Quick recap
- A wave transfers energy, not matter.
- Mechanical waves need a medium; electromagnetic waves don’t.
- Transverse waves move up and down; longitudinal waves move back and forth.
- Waves have wavelength, amplitude and frequency.
- Wave speed = frequency × wavelength.