Waves: Basics
- Waves transfer energy between points, without transferring matter:
- When a wave travels between two points, no matter actually travels with it: The points on the wave simply vibrate back and forth about fixed positions
- The wavelength of a wave is the distance from a point on one wave to the same point on the next wave. Usually this is measured from the top of one wave to the top of the next wave
Wavelength is usually measured in metres (a distance) - The amplitude of a wave is its height, measured from the middle of the wave to its top (or from the middle to its bottom)
Diagram showing the amplitude and wavelength of a transverse wave
- Wavefronts are a useful way of picturing waves from above: each wavefront is used to represent a single wave
Diagram showing a wave drawn as a series of wavefronts
- The frequency of a wave is the number of waves passing a point (or being created or received) every second – it is helpful to think of it as being the waves per second
- The units of frequency are hertz (Hz)
Exam Tip
When labelling wavelength on a diagram, make sure that your arrows go from the very top of a wave to the very top of the next one: if your arrow is too short you will lose marks.
Longitudinal & Transverse Waves
- Waves can exist as one of two main types:
- Transverse
- Longitudinal
Transverse Waves
- For a transverse wave, the points along the wave vibrate at 90 degrees to the direction in which the wave is moving (the direction of energy transfer)
With a transverse wave, the vibrations are at 90 degrees to the direction of energy transfer
Longitudinal Waves
- For a longitudinal wave, the points along the wave vibrate in the same direction that the wave is moving in
With a longitudinal wave, the vibrations are parallel to the direction of energy transfer
Exam Tip
If asked to describe the difference between transverse and longitudinal waves, sketch the above diagrams. A good, clearly labelled diagram can earn you full marks.
The Wave Equation
- The speed of a wave (v) is related to the frequency (f) and wavelength (λ) by the equation:
- You can rearrange this equation with the help of the formula triangle:
Use the formula triangle to help you rearrange the equation
Exam Tip
When stating equations make sure you use the right letters:
Eg. λ for wavelength, not L or W.
If you can’t remember the correct letters, then just state the word equations.
Be careful with units: wavelength is usually measured in metres and speed in m/s, but if the wavelength is given in cm you might have to give the speed in cm/s.
Likewise, watch out for frequency given in kHz: 1 kHz = 1000 Hz
