KS3 · Physics

Water waves

Waves can roll right across a pond, one after another, and yet the pond's water hasn't gone anywhere. So what exactly just crossed the pond?

Lift one bit of water. Watch the wave.

02468-40-2002040Distance along the water (m)Height above rest position (cm)-20 cmmarked waterlift this water
Wave has moved forward 0 m

Wave has moved forward: 0 m. marked water: -20 cm

A model water wave, seen side-on and frozen so you're in control. The dot is one marked bit of water, 3 m along. It starts in a trough. Drag it up to the next peak and keep your eye on the whole wave, then check the number at the top: how far has the wave moved, and how far sideways has your dot gone?

Check your thinking

The floating ball

A small ball floats on a calm pond. Someone makes waves at one end, and they roll under the ball towards the far bank. The waves are smooth: none of them has broken into a foamy, crashing top.

What happens to the ball as the waves pass? Pick the idea closest to what you think.
How sure are you?

When waves meet

Two waves, one spot, one moment

Choose what meets what, then follow the path to the resultant wave.

What meets what? → Same amplitude?

  • The water feels both waves at once. The wave you end up with is called the resultant wave.

4 possible meetings.

On Two waves arrive at the same point at the same moment. 3 branches to choose from.

Two sets of waves can cross the same patch of water. Where they meet, the water gets pushed by both waves at once. Follow each path to see what you get.

Predict, then check

Commit to an answer before you reveal it.

Two waves cross a pond from opposite sides. Where they meet, a peak and a trough of equal amplitude cancel, and for an instant the water there is flat. What happens next?

Physics · Wavefront diagrams

Waves reflecting from a barrier, seen from above
hard barriernormalwavelengthir

View: Reflecting. Showing 5 layers: Barrier and normal, Incoming waves, Wavelength, Reflected waves, Angles

View

Layers

Explore

Switch view · tap a part

The full reflection. The reflected wavefronts keep the same wavelength, and near the barrier the two sets overlap in a criss-cross.

Switch between Arriving and Reflecting, then tap each part to see what it means.

Your turn

Draw a reflection diagram

Waves roll towards a hard, flat wall, arriving 35° from the normal. Build their reflection diagram step by step.

  1. Sketch the incoming wavefronts: straight lines, evenly spaced.
  2. Add an arrow across them showing which way the waves travel, running into the barrier.
  3. missing step
Which line is step 3?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

What actually travels when a wave crosses water, and what happens when waves meet each other or hit a wall.

What you need to know

  • A water wave is an undulation, a shape that travels through the water. Each bit of water oscillates (moves up and down) in place and isn't carried forward, unless the wave breaks on a beach.
  • Water waves transfer energy, not water, in the direction the wave travels. The water oscillates at 90° to that direction, so water waves are transverse.
  • When two waves meet they superpose, adding up or cancelling out to make a resultant wave. Afterwards they pass through each other unchanged.
  • Water waves reflect from a hard barrier: the angle of reflection equals the angle of incidence, and the wavelength stays the same.

The big picture

A water wave is a travelling shape. The water bobs up and down in place while the wave, and its energy, move forward, which makes water waves transverse. A wave has peaks, troughs, a rest position and an amplitude. When two waves meet they superpose, adding up or cancelling, and then carry on unchanged. At a hard barrier they reflect: the angle of reflection equals the angle of incidence.

Key points

1Peak (or crest): the highest point of a wave. Trough: the lowest point. Rest position: the level of the calm, flat water.
2Amplitude: the greatest distance the water moves above or below the rest position. The water goes as far above rest as it goes below.
3School physics usually treats amplitude as constant. In a real water wave it shrinks slightly as the wave travels, because some energy spreads out into the surrounding water and air.
4Superposition: peak + peak gives a larger peak; trough + trough gives a deeper trough; peak + trough of equal amplitude cancel completely; with different amplitudes they only partly cancel.
5Wavefronts are parallel lines, each marking a peak. The gap between neighbours is the wavelength. A wavefront diagram doesn't show amplitude.
6Laws of reflection: the angle of reflection equals the angle of incidence, the two angles are on opposite sides of the normal, and they're in the same plane.

Worked example

Problem

A buoy floats on the sea as smooth, unbroken waves pass under it. At each peak it is lifted 15 cm above the level of the calm water. (a) How far below the calm level does it drop in each trough? (b) What is the amplitude of the waves? (c) How far does the buoy move from the top of a peak to the bottom of a trough?

⚠ Watch out

Thinking the water travels along with a wave. Before a wave breaks, each bit of water only moves up and down in place. It's the wave's shape, and its energy, that move forward.

🧠

Memory hook

The wave goes; the water bobs. And when two waves cancel, they're only hiding: a moment later, both are back.

✓

Check yourself

Without scrolling up: sketch a side-on wave, labelling a peak, a trough, the rest position and the amplitude. Add arrows for how the water moves and where the wave goes. What angle is between them?

Flashcards

(13)
What is a water wave?
An undulation: a shape that travels through the water, while the water itself stays put.
What does each bit of water do as an unbroken wave passes?
It oscillates, moving up and down in place. It isn't carried forward.
What moves forward in a water wave?
The wave's shape (its pattern) and its energy. Not the water.
What happens to a floating object as unbroken waves pass under it?
It bobs up and down. It isn't carried along with the waves.
When does some water really move along with a wave?
After the wave breaks on a beach: then some water moves up the beach.
What is an oscillation?
A repeated back-and-forth (or up-and-down) movement.
What makes a water wave transverse?
The water oscillates at 90° (at right angles) to the direction of energy transfer. A wave that oscillates parallel to that direction isn't transverse.
Define peak, trough and rest position.
Peak: the highest point of a wave. Trough: the lowest point. Rest position: the level of the calm, flat water.
What is the amplitude of a water wave?
The greatest distance the water moves above or below the rest position.
Does a real water wave keep the same amplitude as it travels?
Not quite. It shrinks slightly, as some energy spreads out into the surrounding water and air. School physics usually treats it as constant.
What is superposition?
When two waves arrive at the same point at the same moment, they add up or cancel out. The wave they make is the resultant wave.
What does a wavefront show, and what is the gap between wavefronts?
Each wavefront marks the position of a peak. The gap between neighbouring wavefronts is the wavelength.
What are the laws of reflection?
Angle of reflection = angle of incidence; the angles are on opposite sides of the normal; and they're in the same plane.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

Learning with Lightbulb is opening soon

You can use this lesson now. Join the waitlist and we'll let you know when the full Lightbulb experience is ready.

Keep me posted

More KS3 Physics topics

See the full KS3 Physics curriculum →

How this lesson was checked. This KS3 Physicslesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 30 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.