Properties of wavesIB MYP Sciences: 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. A cork floating on a pond bobs up and down as ripples pass, but it does not travel with the ripples. The water particles vibrate around a fixed position and pass the energy on.
Examples include water waves, sound, light and waves on a rope.
Waves do not carry water (or air) along with them. The medium vibrates in place.
Section 2
Transverse and longitudinal waves
In a transverse wave the vibrations are at right angles to the direction the wave travels. Light and water waves are transverse, and so are waves on a rope shaken up and down.
In a longitudinal wave the vibrations are parallel to the direction of travel. Sound is longitudinal: air particles are pushed together (compressions) and pulled apart (rarefactions) along the line the wave moves.
Think of a slinky: shake it up and down for a transverse wave, push and pull it along its length for a longitudinal wave.
Section 3
Describing a wave
- Amplitude: the maximum distance a point moves from its rest position. Bigger amplitude means more energy.
- Wavelength (λ): the distance from one point on a wave to the same point on the next wave, such as crest to crest. It is measured in metres (m).
- Frequency (f): the number of complete waves passing a point each second. It is measured in hertz (Hz); 1 Hz is one wave per second.
- Period (T): the time for one complete wave, in seconds. T = 1 ÷ f.
Amplitude is measured from the rest position to a crest, not from a trough to a crest.
Section 4
The wave speed equation
The speed of a wave depends on how many waves pass each second and how long each one is:
wave speed (m/s) = frequency (Hz) × wavelength (m), or v = f × λ
Worked example: a wave has a frequency of 5 Hz and a wavelength of 0.20 m. v = 5 × 0.20 = 1.0 m/s.
To find frequency, rearrange: f = v ÷ λ. Always convert wavelengths in cm or km into metres first.
Section 5
Measuring wave speed (criteria B and C)
Wave speed can be found by measuring a distance and a time: speed = distance ÷ time. For water waves in a channel, time a wave over a measured distance with a stopwatch.
Good practice: repeat each measurement and take a mean, keep control variables the same (distance, same channel), and change only one independent variable (such as water depth). The dependent variable is what you measure (time or speed). Human reaction time makes stopwatch readings uncertain, so longer distances or electronic timing improve the results.
Must know
- Waves transfer energy without transferring matter
- Transverse waves (light, water) vibrate at right angles; longitudinal waves (sound) vibrate parallel
- Amplitude, wavelength, frequency (Hz) and period (T = 1 ÷ f)
- v = f × λ, with v in m/s, f in Hz and λ in m
- To measure speed: repeat, take a mean, control variables
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Properties of waves
- A buoy floats on the sea off the coast of Portugal. As waves pass, the buoy bobs up and down but stays in almost the same place. An oceanographer counts 12 wave crests passing the buoy in 60 seconds.Calculate the frequency of the waves and state their period.2 marks
- A student in Nairobi uses a dipper that vibrates 8 times every second to make ripples in a shallow water tank. The ripples have a wavelength of 3.0 cm.Calculate the speed of the ripples. Give the unit.2 marks
- Students in Singapore investigate how the depth of water affects the speed of waves. In a long, shallow channel they make a single wave at one end and use a stopwatch to time how long it takes to travel 2.0 m to the far end. Each depth is tested three times. At a depth of 2 cm the times are 4.6 s, 4.4 s and 4.5 s. At a depth of 4 cm the times are 3.3 s, 3.1 s and 3.2 s.State the independent variable, the dependent variable and one control variable in this investigation.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).