Waves transfer energy from one place to another without transferring matter. Keep that idea in mind and the rest of the topic makes much more sense.
Transverse vs longitudinal
- Transverse waves: the oscillations are perpendicular to the direction of energy transfer. Examples: ripples on water and all electromagnetic waves, including light.
- Longitudinal waves: the oscillations are parallel to the direction of energy transfer. They have compressions and rarefactions. Example: sound waves.
Key words
- Amplitude: the maximum displacement from the rest position.
- Wavelength (λ): the distance from one point on a wave to the same point on the next wave.
- Frequency (f): the number of waves passing a point each second, measured in hertz (Hz).
- Period (T): the time for one complete wave. T = 1 ÷ f
The wave speed equation
wave speed = frequency × wavelength, or v = f × λ
Worked examples
1. A wave has a frequency of 50 Hz and a wavelength of 0.4 m. Find its speed.
v = 50 × 0.4 = 20 m/s
2. Sound travels at 340 m/s. Find the wavelength of a 680 Hz note.
Rearrange: λ = v ÷ f = 340 ÷ 680 = 0.5 m
Top tips
- Watch for wavelengths given in cm or frequencies in kHz and convert them first.
- When measuring wavelength from a diagram, measure across several waves and divide. It’s more accurate.
- Remember it’s the energy that travels, not the water or air.
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