The type of wave depends on one thing: the direction of vibration compared with the direction the wave travels. At right angles is transverse. Along the same line is longitudinal.
This lesson sits inside wave behaviour. It pairs with connecting wavelength, frequency and speed, because the wave equation is used in exactly the same way for both types.
How do the two types move?
In a transverse wave, the vibration is at 90° to the direction of travel. A rope shaken up and down sends a wave along the rope while each piece of rope moves up and down. Light and other electromagnetic waves are transverse, and so are surface water ripples.
In a longitudinal wave, the vibration is parallel to the direction of travel. A push on the end of a stretched spring creates a compression, where coils are close together, followed by a rarefaction, where they are spread out. Sound in air is longitudinal.
In both cases the wave transfers energy without the material travelling along with it.
How do I find the wavelength of each type?
For a transverse wave, the wavelength is crest to crest or trough to trough. For a longitudinal wave, it is compression to the next compression (or rarefaction to the next rarefaction).
Measure from the same point on each repeat. Then use v = f × λ as usual.
Worked example
Invented data: a sound wave passes through air at 340 m/s. The centres of two successive compressions are 0.85 m apart. Find the frequency and state which way the air particles vibrate.
Step 1, wavelength: the gap between successive compressions is one wavelength, λ = 0.85 m.
Step 2, rearrange: f = v ÷ λ.
Step 3, calculate: f = 340 ÷ 0.85 = 400 Hz.
Step 4, direction: sound is longitudinal, so the air particles vibrate back and forth along the direction the sound travels.
Check: 400 × 0.85 = 340 m/s, which matches.
The mistake to watch for
Students often say that in a longitudinal wave the particles move along with the wave, from the source to the listener.
Mistaken answer: “Air particles travel from the speaker to my ear in a sound wave.”
The particles only oscillate a small distance about their rest positions. The pattern of compressions moves on, not the particles.
The correction is to use the phrase “the wave transfers energy, not matter”. Another slip is to draw a sine curve for sound and call it “up and down”. That graph plots displacement, which for sound is actually along the direction of travel.
The bounds and rounding explainer helps when a measured gap such as 0.85 m has limited precision and you want to see the effect on the frequency.
Check yourself
1. Classify each as transverse or longitudinal: sound in air, light, ripples on a pond.
Show answer
Sound in air is longitudinal. Light is transverse. Surface ripples are transverse.
2. The centres of the first and fourth compressions along a spring are 1.2 m apart. Find the wavelength.
Show answer
From the first to the fourth compression is 3 wavelengths. λ = 1.2 ÷ 3 = 0.40 m.
3. A student says that floating cork moves along with the ripple. Correct the statement.
Show answer
The cork moves up and down about its position while the ripple travels on. The wave transfers energy, not the water or cork along the surface.
Where this leads next
Next, practise reading a wave diagram when the scale is not given in ripple-style diagrams without assuming scale. Sound is developed further in electromagnetic spectrum and sound.
If wave diagrams still blur together, a teacher watching you sketch them can correct the habit quickly. That is the kind of help online one-to-one Physics tuition offers.