Transverse and longitudinal waves
All waves transfer energy, but they come in two types depending on how the particles or fields move compared with the wave's direction of travel.
- Transverse wave
- The vibration is at right angles (perpendicular) to the direction the wave travels. Examples: light, all electromagnetic waves, and ripples on water.
- Longitudinal wave
- The vibration is along the same line as the direction of travel, making regions of compression (squashed together) and rarefaction (spread apart). Example: sound.
Describing a wave
Four measurements describe any wave.
- Wavelength (\( \lambda \))
- The length of one complete wave, for example crest to crest, measured in metres, \( \text{m} \).
- Amplitude
- The maximum distance a point moves from its rest position. Bigger amplitude means more energy. Measured in metres, \( \text{m} \).
- Frequency (\( f \))
- The number of complete waves passing a point each second, measured in hertz, \( \text{Hz} \).
- Period (\( T \))
- The time for one complete wave to pass, measured in seconds, \( \text{s} \). It is the reciprocal of frequency.
Frequency and period are partners
If \(5\) waves pass each second, the frequency is \(5\ \text{Hz}\) and each one takes \( T = \tfrac{1}{5} = 0.2\ \text{s} \). Double the frequency and you halve the period.
The wave equation
Wave speed, frequency and wavelength are linked by one equation you will use again and again.
Here \(v\) is the wave speed in \( \text{m/s} \), \(f\) is the frequency in \( \text{Hz} \), and \( \lambda \) is the wavelength in \( \text{m} \).
A sound wave has a frequency of \(170\ \text{Hz}\) and a wavelength of \(2\ \text{m}\). Find its speed.
Reflection
When a wave hits a barrier it can bounce back; this is reflection. The wave obeys the law of reflection: the angle of incidence equals the angle of reflection, both measured from the normal (a line at right angles to the surface). Reflected sound waves are what we hear as an echo.
Properties of sound waves
Sound is a longitudinal wave made by a vibrating object pushing on the particles of a medium. This leads to a few important properties.
- Sound needs a medium (solid, liquid or gas). It cannot travel through a vacuum, so there is no sound in space.
- Sound travels fastest in solids (particles close together), slower in liquids, and slowest in gases.
- A larger amplitude sounds louder; a higher frequency sounds higher in pitch.
- Humans can hear frequencies from about \(20\ \text{Hz}\) to \(20\,000\ \text{Hz}\).
Where this is assessed
Rearranging \( v = f\lambda \) and substituting with correct units is Criterion C (processing). Explaining why sound cannot cross a vacuum, using the particle idea, shows Criterion A (knowing and understanding).
Check yourself
1. Is sound a transverse or a longitudinal wave, and what does that mean? +
Sound is longitudinal: the particles vibrate back and forth along the same direction the wave travels, making compressions and rarefactions.
2. A water wave has a frequency of \(4\ \text{Hz}\) and a wavelength of \(0.5\ \text{m}\). Find its speed. +
Use \( v = f\lambda = 4 \times 0.5 = \mathbf{2\ \text{m/s}} \).
3. A wave has a frequency of \(50\ \text{Hz}\). What is its period? +
Use \( T = \dfrac{1}{f} = \dfrac{1}{50} = \mathbf{0.02\ \text{s}} \). Each complete wave takes two hundredths of a second.
Part of the Physics library. Spotted an error or want a topic added? That feedback makes the notes better.