Waves and sound
General wave properties, the wave equation, reflection, refraction and diffraction, and the properties of sound.
📌 Key points
- Waves transfer energy without transferring matter.
- Transverse waves vibrate at right angles to the direction of travel (light, all electromagnetic waves, water ripples, seismic S-waves).
- Longitudinal waves vibrate along the direction of travel (sound, seismic P-waves); they have compressions and rarefactions.
- Amplitude is the maximum displacement from the rest position; wavelength is the distance between two neighbouring crests; frequency is the number of waves per second (Hz).
- Wave speed = frequency × wavelength: v = fλ.
- Waves can be reflected, refracted (they change speed and direction at a boundary, e.g. water waves slow down in shallow water) and diffracted (they spread out through a gap or around an edge).
- Diffraction is greatest when the gap is about the same size as the wavelength.
- Sound is made by vibrating objects and needs a medium; it cannot travel through a vacuum. Its speed in air is about 330–350 m/s, and it is usually faster in liquids and solids.
- Humans hear from about 20 Hz to 20 000 Hz. Ultrasound is above 20 kHz. A louder sound has a larger amplitude; a higher pitch has a higher frequency.
- An echo is a reflected sound. The sound travels there and back, so distance to the reflector = speed × time2.
📖 Key terms
- Amplitude
- The maximum displacement of a point on a wave from its rest position.
- Wavelength
- The distance between a point on one wave and the same point on the next wave.
- Frequency
- The number of waves passing a point per second; measured in hertz (Hz).
- Wavefront
- A line joining points on a wave that are in the same position, e.g. along a crest.
- Transverse wave
- A wave whose vibrations are perpendicular to its direction of travel.
- Longitudinal wave
- A wave whose vibrations are parallel to its direction of travel.
- Compression
- A region of a longitudinal wave where particles are closer together.
- Ultrasound
- Sound with a frequency above 20 kHz, too high for humans to hear.
✏️ Worked example
A ship sends an ultrasound pulse to the seabed. The echo returns after 0.80 s. Sound travels at 1500 m/s in seawater. How deep is the sea?
- Distance travelled by the pulse = speed × time = 1500 × 0.80 = 1200 m.
- The pulse goes down and back up, so the depth is half of this.
- Answer: depth = 1200 ÷ 2 = 600 m.
In echo questions, remember to halve the distance — the sound goes to the reflecting surface and back again.
Test yourself8 questions from a bank of 22. A new mix every time.
Start the quiz →