Properties of wavesIB MYP Physics: Revision notes
Section 1
Waves transfer energy, not matter
A wave is a regular disturbance that transfers energy from one place to another without transferring matter. Particles of the medium vibrate about fixed positions, and pass on the energy.
A cork on a pond just bobs up and down as ripples pass; the water does not travel with the wave. Sound waves in air and light waves are also examples.
Waves transfer energy, not matter. The particles do not travel along with the wave.
Section 2
Transverse and longitudinal waves
In a transverse wave the vibrations are at right angles to the direction the wave travels. Examples: water ripples, light and other electromagnetic waves, and waves on a rope.
In a longitudinal wave the vibrations are parallel to the direction the wave travels. The particles form regions that are squashed together (compressions) and spread apart (rarefactions). Examples: sound waves and waves along a stretched spring.
Section 3
Describing a wave
On a diagram of a transverse wave drawn as a line above and below a central rest line:
- Crest: the highest point; trough: the lowest point.
- Amplitude: the maximum displacement of the wave from the rest position, measured from the middle line to a crest (or trough). A larger amplitude means more energy.
- Wavelength (λ): the distance from one point on a wave to the same point on the next, e.g. from crest to crest. Measured in metres.
- Frequency (f): the number of complete waves passing a point each second, in hertz (Hz).
- Period (T): the time for one complete wave, in seconds.
Amplitude is measured from the rest line to the crest, not from the trough to the crest.
Section 4
Frequency and period
Frequency and period are linked:
T = 1 ÷ f and f = 1 ÷ T
Example: a wave has a frequency of 5.0 Hz, so its period is T = 1 ÷ 5.0 = 0.20 s. A wave with a period of 0.50 s has a frequency of 1 ÷ 0.50 = 2.0 Hz.
Section 5
The wave equation
The speed of a wave is
v = f × λ
- v = wave speed in metres per second (m/s)
- f = frequency in hertz (Hz)
- λ = wavelength in metres (m)
Rearranged: f = v ÷ λ and λ = v ÷ f.
Example: a wave has frequency 50 Hz and wavelength 6.0 m. Its speed is v = 50 × 6.0 = 300 m/s.
If the wave speed stays the same, doubling the frequency halves the wavelength.
Convert units first: centimetres to metres and kilohertz to hertz, before using v = f λ.
Section 6
Measuring wave speed in a ripple tank
A ripple tank is a shallow tray of water. A motor makes a bar vibrate at a known frequency, producing straight waves.
To find the speed:
- measure the distance across several wavelengths with a ruler (e.g. 5), then divide to find one wavelength
- the frequency is known from the motor
- calculate v = f λ
Measuring across several wavelengths reduces the percentage error in the wavelength. Repeating and taking a mean improves reliability.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Properties of waves
- A cork floats on the surface of a pond in Kerala. A stone is dropped into the pond and ripples spread outwards. As the ripples pass, the cork bobs up and down but stays in about the same place.State how the direction of the vibrations differs between transverse and longitudinal waves, and give one example of each type.2 marks
- A wave machine in a swimming pool in Singapore produces regular water waves with a frequency of 0.50 Hz and a wavelength of 3.0 m.The wave speed in the pool stays at 1.5 m/s. The frequency of the wave machine is increased to 1.0 Hz. Calculate the new wavelength.2 marks
- A student uses a ripple tank to measure the speed of water waves. A motor makes a straight bar dip into the water 8.0 times each second, which produces a train of waves. The student holds a ruler beside the pattern and measures a distance of 12.5 cm across 5 complete wavelengths.Calculate the wavelength and the speed of the water waves.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).