GCSE Physics Waves and sound revision

A wave is a moving disturbance that transfers energy without transferring matter. Drop a stone in a pond and the ripple races outward — but each water particle simply oscillates about its rest position. In transverse waves the oscillations are at right angles to the direction of travel (water ripples, light, S-waves); in longitudinal waves the oscillations are along the direction of travel, making compressions and rarefactions (sound, P-waves). The four measurements of any wave: amplitude (maximum displacement from rest — sets loudness or brightness), wavelength λ in metres (one full cycle), frequency f in hertz (waves passing a point each second), and period T in seconds, with T = 1/f — a 25 Hz wave has period 1 ÷ 25 = 0.040 s.

Every wave obeys v = f λ: speed in m/s, frequency in Hz, wavelength in m. Sound at 512 Hz in air (v ≈ 340 m/s) has wavelength λ = v/f = 340 ÷ 512 ≈ 0.66 m — about the width of your shoulders. The required practicals measure wave speed twice. In the ripple tank, count ripples passing a point for the frequency, photograph the ripples against a scale for the wavelength, and multiply. With a stretched string driven by a vibration generator, adjust the frequency until clear standing waves appear; the string length and number of loops give the wavelength.

Sound is longitudinal: a vibrating object squeezes and stretches the air into compressions and rarefactions. It cannot travel through a vacuum (a bell in a sealed jar falls silent as the air is pumped out) and crosses air at about 340 m/s. People hear roughly 20 Hz to 20 kHz; the upper limit falls with age. Pitch tracks frequency (more vibrations per second = higher note) and loudness tracks amplitude (bigger oscillations = more energy). Ultrasound — above 20 kHz — reflects well off boundaries between different media, so pulses can be ranged: sonar depths, foetal scans and flaw detection all time the echo and use distance = speed × time ÷ 2.

Reflection: a wave bouncing off a boundary keeps its speed and frequency, and the angle of incidence equals the angle of reflection (measured from the normal). Refraction: when a wave crosses into a medium where its speed differs, the wave changes direction — ripples slow in shallower water and bend towards the normal. Waves also carry information about the planet: earthquakes launch P-waves (longitudinal) and S-waves (transverse). P-waves travel through solids and liquids; S-waves travel through solids only. Seismometers on the far side of the Earth receive P-waves but no direct S-waves — an S-wave shadow — because the molten outer core is liquid. P-waves bending suddenly at the core boundary complete the picture of a layered Earth.