Compare transverse and longitudinal waves
Distinguish wave types by oscillation direction and interpret their displacement patterns in diagrams and graphs.
Section 7 · Waves
AS Level; Cambridge International Physics 9702, 2025–2027 syllabus, Topic 7. These original wave examples and diagrams are conceptual study aids; full timed assessment and human subject-teacher review remain outstanding.
Before you begin
- Use frequency, period, wavelength, amplitude, speed and trigonometric ratios with consistent units.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
How do particles oscillate in a transverse wave?
What you will learn
- Compare transverse and longitudinal wave oscillations
- Identify displacement, direction of travel and wavelength on representations
- Interpret graphical depictions of both wave types
Classify waves by oscillation direction
In a transverse wave, the oscillations are perpendicular to the direction of energy transfer. In a longitudinal wave, oscillations are parallel to the direction of energy transfer, producing compressions and rarefactions.
A rope wave is a familiar transverse example; sound in air is longitudinal. These descriptions refer to the oscillation relative to wave travel, not to the wave's shape on a page.
Interpret diagrams using the chosen axis
A transverse-wave graph often shows displacement against position at one instant. Crests and troughs are maximum displacements; adjacent crests are separated by one wavelength.
A longitudinal-wave diagram may show particle positions or density against position. Compressions and rarefactions repeat each wavelength. A sine-like graph can represent pressure variation or longitudinal displacement, so read the labelled vertical quantity rather than assuming it is a transverse shape.
Pause and explain
A compression travels along a spring, and coils oscillate along the same axis. What type is the wave?
Worked example
A spring wave travels to the right while its coils move back and forth along the spring. Classify it and explain.
Show the worked solution
- The wave travels right along the spring.
- The coils oscillate parallel to that direction.
- Parallel oscillation identifies a longitudinal wave.
Answer Longitudinal: oscillation is parallel to the direction of wave travel.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A rope pulse travels horizontally while each point moves vertically. Classify the wave using the two directions.
Give me a hint
Compare point motion with the direction the pulse advances.
Compare my reasoning
- Wave travel is horizontal.
- Point oscillation is vertical.
- The directions are perpendicular.
Transverse wave: point motion is perpendicular to the direction of wave travel.
Look for these in your work
- Travel direction identified
- Oscillation direction identified
- Relationship used to classify
A longitudinal wave diagram shows two adjacent compressions 0.80 m apart. What is the wavelength?
Give me a hint
One wavelength spans one complete repeating pattern, such as compression to the next compression.
Compare my reasoning
- Identify equivalent points in the repeating pattern.
- Adjacent compressions are in phase.
- Their separation equals one wavelength.
Wavelength = 0.80 m.
Look for these in your work
- Pattern repetition identified
- Like points used
- Distance stated as wavelength
A graph shows pressure variation against position as a sinusoidal curve. Explain why the curve alone does not prove the wave is transverse.
Give me a hint
Read what the vertical axis represents before inferring particle motion.
Compare my reasoning
- The vertical axis is pressure, a scalar variation in a longitudinal sound wave can also be plotted against position.
- The curve is a graph representation, not a literal path traced by particles.
- Wave classification requires comparing oscillation direction with propagation direction.
The sinusoidal shape only represents pressure variation; it does not show the particle oscillation direction. A longitudinal wave can produce this graph, so classification needs the axis labels and direction relationship.
Look for these in your work
- Graph meaning interpreted
- Shape not mistaken for particle path
- Direction criterion stated
Common mistakes
- Classifying from a drawn curve without checking what the axes represent
- Calling every wave in a material transverse
- Confusing particle oscillation direction with energy-transfer direction
What can you explain now?
How do particles oscillate in a transverse wave?
Compare with the explanation
Perpendicular to wave travel
The oscillation is at right angles to the energy-transfer direction.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Why must you inspect a wave graph's labels before classifying the wave?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.