IGCSE Physics Waves & sound notes

A wave transfers energy without transferring matter. Flick one end of a rope and a hump runs down it: each piece of rope only moves up and down, but the energy reaches the far end. In a water wave a floating duck bobs in place as the ripples pass — the wave moves, the duck does not travel with it.

The vocabulary of every wave: the wavefront is a line or surface marking wave crests moving together (like the front of a ripple); the wavelength λ is the distance between one crest and the next (one full cycle); the amplitude is the maximum displacement from the undisturbed position — half the height from crest to trough; the frequency f counts waves passing a point each second, in hertz; the crest (peak) and trough are the highest and lowest points of a transverse wave.

A ripple tank is a shallow tray of water with a wave generator; a lamp above projects the ripples as bright and dark lines onto the floor below, so wavefronts can be watched and measured.

• Reflection: straight wavefronts hitting a plane (flat) barrier bounce away at the same angle, keeping their wavelength and speed.
• Refraction: where the water becomes shallower, the waves slow down; the wavefronts bunch closer (shorter wavelength) and the direction of travel bends towards the normal. Speed change causes refraction.
• Diffraction through a gap: as waves pass through an opening they spread into the space beyond, curving around the sides.
• Diffraction at an edge: waves passing the end of a barrier curl around behind it.

Supplement: diffraction is strongest when the gap width is similar to the wavelength. A wide gap lets the waves pass almost straight with only slight curving at the edges; a narrow gap (about one wavelength) turns the waves into near-circular spreading beyond it. At an edge, longer wavelengths diffract more — they curl further around the corner, which is why low-pitched bass sound is heard around a building’s corner better than treble.

Sound is produced by vibrating sources: a drum skin shaking, a guitar string twanging, a vocal cord buzzing. The vibrating surface pushes and pulls the air around it, sending out a longitudinal wave: regions where the air is squashed together (compressions) alternate with stretched-out regions (rarefactions) — Supplement. The particles vibrate back and forth along the same line the sound travels.

Sound needs a medium. In a vacuum there is nothing to compress, so no sound crosses it — the classic ringing-bell-in-a-vacuum-jar demonstration fades to silence as the air is pumped out. Human ears hear frequencies from about 20 Hz to 20 000 Hz; below that is infrasound, above it ultrasound.

In air, sound travels at approximately 330–350 m/s. A simple measurement: stand a measured distance d from a large wall, clap in rhythm with the echo, and time, say, 30 clap-to-echo intervals with a stopwatch; the sound travels out and back each time, so v = 2 × d × (number of claps) ÷ total time. Electronic timing with microphones and a data logger is more accurate than clapping. Supplement: sound generally travels faster in solids than liquids, and faster in liquids than gases — the particles are closer together, so the vibration passes on sooner.

Loudness and pitch: a bigger amplitude sounds louder; a higher frequency sounds higher in pitch. A quiet, deep hum has small amplitude and low frequency; a loud, shrieking whistle has large amplitude and high frequency.

An echo is simply the reflection of sound from a hard surface. Because the sound travels to the surface and back, the distance to the surface is half the total distance travelled: distance = (v × t) ÷ 2.

Ultrasound — sound above 20 kHz — is beyond hearing but obeys the same physics, and its short wavelength makes it easy to aim. Its uses (Supplement): non-destructive testing of materials, where cracks inside a metal casting reflect ultrasound before the part ever fails in service; medical scanning of soft tissue, where pulses reflect off boundaries between different tissues to build an image of, say, an unborn baby; and sonar, where a ship times the echo from the seabed to measure the depth of water.