5.1.2Waves
Transverse and Longitudinal Waves
5 min read 5 parts Lesson 2 of 8
Section 1 of 5Transverse Waves
The key idea is the angle between the vibration and the direction the wave travels.
Direction
- In a transverse wave the vibrations are at right angles to the direction in which the wave transfers energy.
Shape
- The wave appears as alternating high points called crests and low points called troughs along the medium.
Examples
- Water ripples, waves on a rope and all electromagnetic waves, including light and radio, are transverse.
Measuring
- Wavelength can be measured from one crest to the next crest, or from one trough to the next trough.
Amplitude
- The amplitude is the maximum displacement from the rest position, measured from the middle line to a crest or trough.
Section 2 of 5Longitudinal Waves
Longitudinal Waves
Direction
- In a longitudinal wave the vibrations are parallel to the direction of energy transfer, backwards and forwards along the line of travel.
Compressions
- Regions where the particles are squeezed close together, giving a higher pressure than normal.
Rarefactions
- Regions where the particles are spread further apart than normal, giving a lower pressure.
Examples
- Sound waves in air and compression waves along a stretched spring are longitudinal waves.
Wavelength
- The distance from the centre of one compression to the centre of the next compression.
Section 3 of 5Comparing the Two Types
A clear comparison helps decide which type of wave is being described.
1
Identify the direction the wave is travelling by finding the direction in which energy is being transferred.
2
Observe the direction of vibration of a single particle, such as one coil of a spring or one floating cork.
3
If the vibration is at 90° to the direction of travel, the wave is transverse and shows crests and troughs.
4
If the vibration is along the line of travel, the wave is longitudinal and shows compressions and rarefactions.
5
Remember that both types still transfer energy only, and neither moves the material of the medium along with it.
| Feature | Transverse | Longitudinal |
|---|---|---|
| Vibration direction | At 90° to travel | Parallel to travel |
| Features | Crests and troughs | Compressions and rarefactions |
| Examples | Light, water ripples, rope waves | Sound, spring compressions |
| Needs a medium? | Not always (light does not) | Yes, always |
Section 4 of 5Worked Example
Question
- A student sends compression waves along a long spring.
- 30 compressions pass a point in 20 s.
- Calculate the frequency and period, and state whether the wave is transverse or longitudinal.
- The coils vibrate along the length of the spring, parallel to the direction of travel, so the wave is longitudinal.
- Frequencyf = N ÷ t = 30 ÷ 20 = 1.5 Hz.
- PeriodT = 1 ÷ f = 1 ÷ 1.5 = 0.67 s (to two significant figures).What the symbols mean: T = period, the time for one complete wave (s) · f = frequency (Hz)
- Check30 waves × 0.67 s is about 20 s.
Answer The wave is longitudinal, with f = 1.5 Hz and T ≈ 0.67 s.
Section 5 of 5Exam Checklist & Tips
5.1.2 – Key Points for the Exam
- Transverse wave A wave in which vibrations are at right angles to the direction of energy transfer.
- Longitudinal wave A wave in which vibrations are parallel to the direction of energy transfer.
- Compression A region of a longitudinal wave where particles are close together.
- Rarefaction A region of a longitudinal wave where particles are spread apart.
- Crest The highest point of a transverse wave.
Common Mistakes
- Stating that sound is a transverse wave because it is drawn as a wavy line on an oscilloscope.
- Mixing up compressions with crests; compressions belong only to longitudinal waves.
- Measuring wavelength from a crest to the following trough, which is only half a wavelength.
- Saying particles travel along with the wave rather than vibrating about a fixed position.