Transverse and longitudinal waves
Waves carry energy from one place to another without carrying the material along. The two kinds differ in which way the particles or fields vibrate compared with the way the wave travels.
Part 1 of 3: Learn it
In short
- Transverse: the vibrations are at right angles to the direction the wave travels.
- Longitudinal: the vibrations are along the direction the wave travels, making compressions and rarefactions.
- Period T = 1 ÷ f: a 50 Hz wave has a period of 0.02 s.
Where this is in your specification
Spec points: AQA 4.6.1.1 and 4.6.1.2 (Combined Trilogy 6.6.1.1 and 6.6.1.2), Edexcel Topic 4, OCR P5.1 (J249) and P4.1 (J250)
| Board | Topic: Waves and the electromagnetic spectrum |
|---|---|
| AQA 8463 | 4.6.1.1, 4.6.1.2 and 4.6.2 (8463); 6.6 (8464) |
| Edexcel 1PH0 | Topics 4 and 5 |
| OCR J249 | P5 (J249), P4.1 and P4.2 (J250) |
| Eduqas C420 | 5.1, 5.2, 6.1, 6.2 |
| Cambridge IGCSE 0625 | 3.1, 3.3 |
| National 5 C857 75 | Waves: wave parameters and behaviours, electromagnetic spectrum |
The two types
| Transverse | Longitudinal | |
|---|---|---|
| Vibration | at right angles to the direction of travel | back and forth along the direction of travel |
| Examples | ripples on water, waves on a string, light and all electromagnetic waves | sound in air, some seismic waves (P-waves) |
| Features | peaks and troughs | compressions (squashed) and rarefactions (spread out) |
Waves move energy, not matter
A ball floating on a pond bobs up and down as ripples pass, but it does not travel across the pond with them. In sound, the air particles vibrate about fixed positions; it is the energy that travels to your ear.
Describing a wave
- Amplitude: the maximum displacement of a point from its rest position. On a diagram, measure from the middle line to a peak, not from peak to trough.
- Wavelength (λ): the distance from a point on one wave to the same point on the next, such as peak to peak.
- Frequency (f): the number of waves passing a point each second, in hertz (Hz).
- Period (T): the time for one complete wave to pass, in seconds.
Period and frequency
So a higher frequency means a shorter period. Wave speed links frequency and wavelength, and has its own notes page.
What kind of wave is light?
Show the answer
Transverse, like all electromagnetic waves.
Part 2 of 3: See it worked
Worked examples
Example 1
A wave has a period of 0.004 s. What is its frequency?
- f = 1 ÷ T
- f = 1 ÷ 0.004
Answer: 250 Hz.
Example 2
A student counts 12 ripples passing a post in 3 seconds. Find the frequency and the period.
- f = 12 ÷ 3 = 4 Hz
- T = 1 ÷ 4 = 0.25 s
Answer: 4 Hz and 0.25 s.
Common mistakes
- Measuring amplitude from a peak to a trough. That is twice the amplitude.
- Calling sound a transverse wave.
- Saying water moves across the pond with the wave.
- Measuring wavelength from a peak to the next trough. That is half a wavelength.
What is a rarefaction?
Show the answer
A region in a longitudinal wave where the particles are spread further apart.
Part 3 of 3: Test yourself
Check yourself
Answer each one in your head or on paper first, then open it to check.
What kind of wave is light?
Transverse, like all electromagnetic waves.
What is a rarefaction?
A region in a longitudinal wave where the particles are spread further apart.
A sound has a frequency of 500 Hz. What is its period?
1 ÷ 500 = 0.002 s.
Jobs that use this
Each link opens the job profile on the National Careers Service (England). In the rest of the UK: My World of Work (Scotland), Careers Wales, nidirect careers (Northern Ireland).
Full lessons and marked practice for this course are coming soon to Brainlag Learn. See courses