How waves act when they meet things
Waves don’t just travel in straight lines forever. When they hit a surface, pass into a new material, squeeze through a gap or meet another wave, they change. These changes are called wave behaviours.
Reflection
Reflection happens when a wave bounces off a surface.
- Light reflecting off a mirror lets you see yourself.
- Sound reflecting off a cliff creates an echo.
- Water waves bounce off the side of a pool.
The law of reflection
The angle at which a wave hits a surface equals the angle at which it bounces off. Both angles are measured from an imaginary line perpendicular to the surface, called the normal.
Uses
| Use | Wave |
|---|---|
| Mirrors | Light |
| Sonar on ships | Sound |
| Bat echolocation | Sound |
| Radar | Radio waves |
| Ultrasound images | Sound |
Refraction
Refraction is the bending of a wave when it passes into a new medium at an angle and changes speed.
Imagine a lawnmower rolling from the sidewalk onto thick grass at an angle. One wheel hits the grass first and slows down, so the mower turns. Waves bend for the same reason.
| Example | Explanation |
|---|---|
| A straw looks bent in water | Light slows and bends when moving from water to air |
| Pools look shallower than they are | Refraction makes the bottom appear higher |
| Lenses in glasses and cameras | Shaped glass bends light to focus it |
| Rainbows | Sunlight refracts in raindrops, splitting into colours |
| Ocean waves curving toward a beach | Waves slow in shallow water and bend |
| Hearing sounds farther away at night | Sound bends back toward the ground through layers of air at different temperatures |
Diffraction
Diffraction is the spreading of a wave around an obstacle or through an opening.
Waves diffract most when the opening is about the same size as the wavelength.
- You can hear someone around a corner because sound waves (about 1 m long) spread through doorways.
- You can’t see around a corner because light waves are tiny compared with a doorway.
- Ocean waves spread out after passing through a gap in a breakwater.
- Long AM radio waves bend around hills better than shorter FM waves, so AM can be heard farther in hilly areas.
Interference
Interference happens when two or more waves meet. Their effects add together.
| Type | What happens | Result |
|---|---|---|
| Constructive interference | Crest meets crest (in step) | A bigger wave |
| Destructive interference | Crest meets trough (out of step) | A smaller wave or no wave |
Examples:
- Noise-cancelling headphones create sound waves exactly opposite to background noise, so they cancel out.
- Dead spots in a concert hall happen where sound waves cancel.
- Soap bubbles shimmer with colours because light waves reflecting from the inner and outer surfaces interfere.
- Rogue waves in the ocean can form when several waves add up.
After passing through each other, waves carry on unchanged.
Standing waves
When a wave reflects back on itself, the two waves can interfere to make a standing wave that seems to stay in place. Guitar strings and organ pipes produce musical notes this way.
Absorption
Absorption happens when a material takes in a wave’s energy, often turning it into heat.
- Dark clothing absorbs light and gets warm.
- Carpets and curtains absorb sound, making rooms quieter.
- Recording studios use foam panels to absorb echoes.
Transmission
Transmission is when a wave passes through a material. Light passes through glass; sound passes through walls (a little).
Common mistakes
- Confusing reflection and refraction. Reflection bounces; refraction bends.
- Thinking waves stop when they meet. They pass through each other.
- Thinking diffraction only happens to sound. All waves diffract.
Try it yourself
- Place a pencil in a glass of water and look from the side. Draw what you see and explain it.
- Bounce a ball off a wall at different angles. Compare the incoming and outgoing angles.
- Make ripples in a tub of water from two points at once. Look for where the waves get bigger and where they cancel.
Quick recap
- Reflection: waves bounce off surfaces.
- Refraction: waves bend when they change speed in a new medium.
- Diffraction: waves spread through openings and around obstacles.
- Interference: waves combine to make bigger or smaller waves.
- Absorption: materials take in wave energy.