General Wave Properties
Section: Waves | Syllabus: Cambridge IGCSE Physics (0625)
Waves and Energy Transfer
A wave is a disturbance that transfers energy from one place to another without transferring matter.
- As a wave travels through a medium, the particles of the medium vibrate about a fixed position - they do not travel along with the wave
- Only energy moves from the source to other points; the matter of the medium stays, on average, in the same place
- Example: a cork floating on water bobs up and down as ripples pass beneath it, but it does not travel outward with the ripples
Demonstrating Wave Motion
Wave motion can be shown using vibrations in ropes and springs, and using water waves.
Vibrations in a Rope or Spring
- Fix one end of a rope or spring and give the free end a single sharp flick, or shake it continuously from side to side
- A pulse (or a continuous wave) travels along the rope, carrying energy from the hand to the fixed end
- Each point on the rope moves briefly away from its rest position and then returns to it - the rope itself does not move along its length
A single pulse travelling along a rope
Water Wave Experiments
- Dip a finger, ruler or dipper repeatedly into a tray or tank of still water at one point
- Circular ripples spread outward from the point of disturbance, carrying energy away from the source
- A small floating object, such as a cork, bobs up and down as the ripples pass but does not travel outward with them - showing that the wave transfers energy, not water
Ripples spreading from a point source on water
Features of a Wave
Wavefront
- A wavefront is a line (or surface) joining points on a wave that are vibrating in phase with each other - for example, a line joining all the points that are at a crest at the same instant
- A wavefront represents the shape of the wave at one instant in time, and the wave travels in the direction perpendicular to its wavefronts
- Waves spreading from a point source have circular wavefronts; waves far from their source, or generated by a straight source, have straight, parallel wavefronts (plane waves)
Crest, Trough and Amplitude
- Crest (peak): the highest point of a wave, above the undisturbed (rest) position
- Trough: the lowest point of a wave, below the undisturbed (rest) position
- Amplitude: the maximum displacement of a point on the wave from its rest position
Wavelength, Frequency and Wave Speed
- Wavelength (λ): the distance between two adjacent points on a wave that are vibrating in phase, such as from one crest to the next, or between corresponding points on two adjacent wavefronts
- Frequency (f): the number of complete waves passing a fixed point per second
- Wave speed (v): the distance travelled by the wave per unit time
Features of a transverse wave
| Feature | Symbol | Unit | Description |
|---|---|---|---|
| Wavefront | - | - | Line joining points on a wave that are in phase |
| Amplitude | A | metre (m) | Maximum displacement from the rest position |
| Wavelength | λ | metre (m) | Distance between two adjacent points in phase |
| Frequency | f | hertz (Hz) | Number of complete waves passing a point per second |
| Wave speed | v | metre/second (m/s) | Distance travelled by the wave per second |
The Wave Equation
- Formula:
- v = fλ
- Where:
- v = wave speed (m/s)
- f = frequency (Hz)
- λ = wavelength (m)
Worked Example: Finding Wave Speed
Museli generates waves in a ripple tank with a frequency of 50 Hz and a wavelength of 2 m. Find the speed of the waves.
- Given: f = 50 Hz, λ = 2 m
- v = fλ
- v = 50 × 2 = 100 m/s
Worked Example: Finding Wavelength
Muchindu measures a sound wave travelling at 330 m/s with a frequency of 440 Hz. Calculate its wavelength.
- Given: v = 330 m/s, f = 440 Hz
- Rearrange: λ = (v/f)
- λ = (330/440) = 0.75 m
Worked Example: Finding Frequency
Muumbe shines light of wavelength 6 × 10^-7 m, travelling at 3 × 10^8 m/s. Find its frequency.
- Given: λ = 6 × 10^-7 m, v = 3 × 10^8 m/s
- Rearrange: f = (v/λ)
- f = 3 × 10^86 × 10^-7 = 5 × 10^14 Hz
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