Waves in air, fluids and solids

Key content

Waves transfer energy (and information) from one place to another without transferring matter.

Transverse and longitudinal waves

• In a transverse wave, the oscillations are perpendicular (at right angles) to the direction of energy transfer. Examples: ripples on a water surface, all electromagnetic waves, S-waves.
• In a longitudinal wave, the oscillations are parallel to the direction of energy transfer. They show areas of compression (particles close together) and rarefaction (particles spread out). Examples: sound waves in air, P-waves.

Evidence that the wave travels, not the medium:

• A cork floating on ripples bobs up and down but does not move along with the wave.
• When sound travels through air, the air particles vibrate back and forth about fixed positions – the air itself doesn't travel to your ear. Evidence: a candle flame or a light piece of paper in front of a loudspeaker vibrates back and forth but is not blown away from it.

Properties of waves

• Amplitude – the maximum displacement of a point on a wave away from its undisturbed position.
• Wavelength ($\lambda$) – the distance from a point on one wave to the equivalent point on the adjacent wave (for example, peak to peak).
• Frequency ($f$) – the number of waves passing a point each second, measured in hertz (Hz).
• Period ($T$) – the time for one complete wave to pass a point.
• Wave speed ($v$) – the speed at which the energy is transferred (or the wave moves) through the medium.

Measuring the speed of sound in air: two people stand a measured distance apart (for example 400 m). One makes a loud sound with a visible action (clapping boards together); the other starts a stopwatch on seeing it and stops it on hearing it. Speed = distance ÷ time. Repeat and take a mean. (Typical result: about 330 m/s.)

Measuring the speed of ripples on water: in a ripple tank, use a strobe or a video to find the wavelength (measure across several waves and divide) and count the waves passing a point in 10 s to find the frequency; then $v = f \lambda$.

You measure frequency, wavelength and speed for waves in a ripple tank and on a stretched string in Required practical 8.

Reflection of waves

At the boundary between two different materials, waves can be:

• reflected – bounced back
• absorbed – energy transferred to the material (which may warm up)
• transmitted – passed through, often being refracted.

What happens depends on the wavelength and on the materials.

In a ray diagram of reflection, the angle of incidence equals the angle of reflection, both measured from the normal (a line at 90° to the surface).

You investigate reflection by different surfaces and refraction by different substances in Required practical 9.

Sound waves

Physics only

Sound waves is physics only and higher tier content.

Sound waves can travel through solids, causing vibrations in the solid.

In the ear, sound waves make the ear drum and other parts vibrate, which causes the sensation of sound. This conversion from sound waves to vibrations of solids only works over a limited frequency range, which sets the limits of human hearing: 20 Hz to 20 kHz. Outside this range the ear drum and the small bones behind it cannot vibrate in response to the sound, so nothing is heard. The range gets narrower with age and with damage to the ear.

Waves for detection and exploration

Physics only

Waves for detection and exploration is physics only and higher tier content.

Differences in velocity, absorption and reflection of waves in solids and liquids can be used to detect and explore structures hidden from direct observation.

• Ultrasound has a frequency above 20 kHz. It is partially reflected at the boundary between two media. The time taken for reflections to return tells us how far away the boundary is. Used for medical imaging (such as pre-natal scans) and industrial imaging (detecting cracks in metal).
• Echo sounding uses high-frequency sound to detect objects in deep water and measure water depth.
• Seismic waves are produced by earthquakes:
  • P-waves are longitudinal and travel through solids and liquids (at different speeds).
  • S-waves are transverse and cannot travel through liquids.
  • The S-wave shadow zone provides evidence that the Earth's outer core is liquid, and the way P-waves refract shows the size of the core.

The study of seismic waves provided new evidence about parts of the Earth that cannot be observed directly.

Equations

Worked examples

Common misconceptions