GCSE · Physics · Edexcel · Spec 1PH0

Longitudinal vs transverse waves

Flick a rope and a bump races away from your hand. Did any rope run off with it? Answer that, and one two-arrow test sorts every wave.

Two waves, two arrows each

The arrow test

Showing 2 layers: The two waves, Travel and oscillation arrows

Layers

Explore

Switch the arrows off, predict them, switch them on.

Two arrows per wave: which way the wave travels, and which way the material oscillates. Right angles on the rope, parallel on the particles.

Hide the arrows, guess where they go, then bring them back. Tap each name to find it on the right wave.

Sort it

Which type? Or both?

Tick Transverse, Longitudinal or both for each item. Some belong in the overlap.

A Transverse
B Longitudinal

Water waves
Light waves
S waves in earthquakes
Sound waves
Ultrasound
Oscillations at right angles to the travel direction
Oscillations parallel to the travel direction
Compressions and rarefactions
Crests and troughs
Transfers energy without transferring matter
Has a wavelength
Has an amplitude

Predict, then check

Look back at the row of air particles in the diagram. Where would you measure a wavelength?

On the row of air particles carrying a sound wave, which distance is one wavelength?

What do you think?

The loudspeaker across the room

A loudspeaker is playing music. You are standing across the room from it, and you can hear it clearly.

Which idea is closest to what you think is happening to the air between the speaker and your ear?

The air near the speaker travels across the room to my ear, carrying the sound with it.Air particles shuffle forwards and backwards about their rest positions and pass the energy along.Sound does not need anything to travel through, so it would reach me just as well across empty space.The air particles move up and down, at right angles to the direction the sound travels.

How sure are you?

I'm sureI think soI'm guessing

The air near the speaker travels across the room to my ear, carrying the sound with it.

This is a common misconception

Easy trap, and the most common one. The air does not make the journey. Each air particle swings forwards and backwards about its own rest position and then ends up roughly where it started. What travels across the room is the energy, handed from particle to particle. Both types of wave transfer energy without transferring matter.

Air particles shuffle forwards and backwards about their rest positions and pass the energy along.

This matches the evidence

Spot on. Because the shuffling is forwards and backwards along the direction the sound travels, the oscillation is parallel to the travel. That makes sound a longitudinal wave, and the bunching and spreading out of the particles gives compressions and rarefactions.

Sound does not need anything to travel through, so it would reach me just as well across empty space.

This is a common misconception

Not quite. A sound wave is particles oscillating and passing energy to their neighbours, so it needs a medium: it can travel through gases, liquids and solids. In a vacuum there are no particles to oscillate, so there is nothing to pass the energy on and sound cannot cross it.

The air particles move up and down, at right angles to the direction the sound travels.

This is a common misconception

That would make sound a transverse wave, and it is not one. Sound is longitudinal: the air particles oscillate forwards and backwards along the direction of travel, not across it. Run the arrow test on the particle row in the diagram. The two arrows are parallel.

A wave is the medium wobbling in place while energy moves on. No medium to wobble means no sound.

Your turn

Say it in your own words

Describe how a transverse wave differs from a longitudinal wave. Give one example of each type of wave. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

In a transverse wave the oscillations are at right angles (perpendicular) to the direction the wave travels. (1 mark)
In a longitudinal wave the oscillations are parallel to the direction the wave travels (forwards and backwards along it). (1 mark)
A correct example of a transverse wave, such as water waves, waves on a rope, light, radio waves or S waves. (1 mark)
A correct example of a longitudinal wave, such as sound or ultrasound. (1 mark)

Model answer

In a transverse wave the oscillations are at right angles to the direction the wave travels, for example in waves on a rope. In a longitudinal wave the oscillations are forwards and backwards, parallel to the direction the wave travels, for example in sound.

Compare the oscillation with the travel for both types, then add one example of each.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

The medium wobbles in place. Whether that wobble is across the travel or along it decides if the wave is transverse or longitudinal.

What you need to know

Waves transfer energy forwards without transferring matter: each part of the medium oscillates about its rest position.
Transverse wave: the oscillations are at right angles to the direction the wave travels. Longitudinal wave: the oscillations are parallel to it, forwards and backwards.
Transverse examples: water waves, waves on a rope, light, radio waves and S waves. Longitudinal example: sound, including ultrasound.
A transverse wave has crests and troughs. A longitudinal wave has compressions and rarefactions.
Wavelength is crest to crest on a transverse wave and compression to compression on a longitudinal wave. Amplitude is the maximum displacement.

The big picture

Every wave is a medium wobbling in place while energy moves on. If the wobble is at right angles to the direction of travel the wave is transverse. If it is along the direction of travel the wave is longitudinal.

Key points

1Run the arrow test: draw the travel arrow and the oscillation arrow. Right angles mean transverse. Parallel means longitudinal.

2In a transverse wave the oscillations can be up and down, side to side or in any direction, as long as they are at right angles to the travel.

3A compression is where the particles are squashed closest together. A rarefaction is where they are most spread out, halfway between two compressions.

4The medium is the material the wave travels through. Sound needs one, so it cannot travel through a vacuum.

5Displacement is how far a part of the wave is from its rest position. Amplitude is the maximum displacement.

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Worked example

Problem

You stretch a slinky along a table and hold one end. You push your hand forwards and pull it back along the slinky's length, over and over, and a wave runs to the far end. Is it transverse or longitudinal? Use the two-arrow test.

⚠ Watch out

Saying that transverse waves go up and down. The oscillation can be up and down, side to side or any direction, as long as it is at right angles to the direction of travel.

🧠

Memory hook

Transverse goes across the travel line, like the crossbar of a T. Longitudinal goes along it, and long means along.

✓

Check yourself

Ultrasound is sound of very high frequency. Is it transverse or longitudinal? Then explain to a friend who says the air rushes from the source to the receiver why that idea is wrong.

Flashcards

(12)

What single comparison separates the two types of wave?

The oscillation direction against the travel direction: at right angles is transverse, parallel is longitudinal.

Which way can the oscillations go in a transverse wave?

Up and down, side to side or any direction, provided it is at right angles to the direction the wave travels.

Which way do the oscillations go in a longitudinal wave?

Forwards and backwards, in the same direction as (parallel to) the direction the wave travels.

What is a compression?

A place in a longitudinal wave where the particles of the medium are squashed most closely together.

What is a rarefaction, and where does it sit?

A place where the particles are most spread out. Each rarefaction is halfway between two compressions.

Name five transverse waves.

Water waves, waves on a rope, light waves, radio waves and S waves in earthquakes.

Give the longitudinal wave example, and the very high frequency version of it.

Sound waves. Ultrasound is sound of very high frequency.

What does a wave transfer, and what does it not transfer?

It transfers energy forwards. It does not transfer matter, because each part of the medium oscillates about its rest position.

What is the wave medium?

The material the wave travels through, such as the rope, the water or the air.

Can sound travel through a vacuum?

No. Sound needs a medium. It can travel through gases, liquids and solids.

Where do you measure wavelength on each type of wave?

Transverse: from one crest to the next. Longitudinal: from one compression (or rarefaction) to the next.

What are displacement and amplitude?

Displacement is how far a part of the wave has moved from its rest position. Amplitude is the maximum displacement.

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.

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Which type? Or both?

The loudspeaker across the room

Say it in your own words

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