GCSE Combined Science Waves & the EM spectrum revision

A wave is a disturbance that carries energy from place to place without carrying matter with it. Drop a stone in a pond and the ripples race outwards, but the floating leaf merely bobs up and down — the water moves vertically, the wave moves outwards. That separation — energy travels, medium oscillates about a fixed point — is the opening mark of most wave questions.

Two families cover every wave you meet. In transverse waves the oscillations are perpendicular (at right angles) to the direction of travel: water ripples, waves on a plucked string, and every electromagnetic wave. In longitudinal waves the oscillations are parallel to the direction of travel — the medium squashes together (compressions) and stretches apart (rarefactions): sound waves, ultrasound, and earthquake P-waves. Push–pull a Slinky along its length and you can watch compressions run down it; shake one end sideways and you make a transverse wiggle instead.

Amplitude is the height of a peak (or depth of a trough) measured from the rest position — it sets the energy carried, which for sound is loudness. Wavelength (λ) is the distance between two matching points on the next wave — crest to crest, compression to compression. Frequency (f) counts the waves passing a point each second, in hertz (Hz): 1 Hz = 1 wave per second. Period (T) is the time for one full wave, and T = 1 ÷ f — a wave of 4.0 Hz has a period of 0.25 s. For sound, frequency is pitch and amplitude is loudness.

The one equation to learn: v = f λ — speed (m/s) = frequency (Hz) × wavelength (m). Keep units matched: if frequency arrives in megahertz, convert to hertz first (1 MHz = 10⁶ Hz); if wavelength arrives in nanometres, convert to metres (1 nm = 10⁻⁹ m). The equation works identically for ripples crawling across a tank at centimetres per second and for gamma rays sprinting at 3 × 10⁸ m/s.

Ripple tank (water waves). A wooden bar dips into shallow water and is vibrated by a motor at a chosen frequency. To measure frequency, count the waves passing a marked point against a stopwatch (divide count by time — 12 waves in 4.0 s gives 3.0 Hz). To measure wavelength, freeze the moving pattern with a stroboscope (or a phone photo): the frozen crests look static, so a ruler laid on the screen measures the crest-to-crest distance — several wavelengths measured at once and divided by the count cuts the error. Multiply to get the speed; at 3.0 Hz and 0.040 m wavelength the ripples crawl at 0.10 m/s.

Stretched string. Tie a string over a pulley to a mass hanger and drive it with a vibration generator on a signal generator. Adjust the frequency until clear loops (standing waves) appear; each loop is half a wavelength, so a length of 1.6 m holding four loops has λ = 1.6 ÷ 4 = 0.40 m. Read the frequency from the signal generator dial and compute v = f λ. Increasing the hanging mass raises the wave speed (a tighter string), which shows up as more loops for the same frequency.

When a wave hits a boundary it can reflect: the angle of incidence equals the angle of reflection, both measured from the normal (the line at right angles to the surface). An echo is exactly this — a sound wave reflecting off a wall. When a wave crosses into a new medium and changes speed, its direction can change too: refraction. Light bends entering glass; water ripples slow and shorten when they cross into shallower water. The frequency never changes at a boundary — the source still emits the same waves per second — so a slower wave must have a shorter wavelength (v = f λ with f fixed).

Sound is longitudinal: vibrating objects (a drum skin, a loudspeaker cone) push and pull the air into compressions and rarefactions that race away at about 330–340 m/s in air. It cannot travel through a vacuum — the classic bell-in-a-vacuum demo falls silent. Human hearing runs from about 20 Hz to 20 kHz; at 340 m/s a 20 Hz wave is 17 m long while a 20 kHz wave is a centimetre-scale 0.017 m. Sound reflects (echoes) and refracts just like any other wave.

Earthquakes launch seismic waves through the planet. P-waves are longitudinal and faster (roughly 8 km/s in the crust and mantle), so they arrive at monitoring stations first; they travel through solids and liquids. S-waves are transverse and slower (roughly 4 km/s) and travel only through solids. Because stations on the far side of the Earth receive P-waves but no direct S-waves, part of the interior must be liquid — the liquid outer core. Comparing arrival times (a 480 km-away quake reaches a station with P-waves in 60 s but S-waves only after 120 s — a 60 s lag) also lets geologists locate the epicentre.

Electromagnetic (EM) waves are transverse ripples in crossed electric and magnetic fields, and they are odd in one wonderful way: they need no medium and all travel at exactly 3 × 10⁸ m/s through a vacuum. They transfer energy from a source to an absorber — the Sun to your skin, a phone mast to your handset. The spectrum is continuous but we slice it into seven named bands in order of increasing frequency (decreasing wavelength): radio, microwaves, infrared, visible light, ultraviolet, X-rays, gamma rays. Visible light is a narrow slice in the middle — remember it as the single band with an obvious everyday detector: your eye.

The hazard column climbs with frequency. Radio and microwaves mostly heat tissue if intense (which is why microwave ovens are shielded); infrared burns skin the way a grill does; ultraviolet damages DNA in skin and eye cells — the physics behind sunscreen and shaded welding visors; X-rays and gamma rays are ionising: their photons carry enough energy to strip electrons from atoms, mutating cells and causing cancer — which is exactly why radiographers step behind lead screens, and also why the same waves can target and kill tumours. Because v is fixed at 3 × 10⁸ m/s, high frequency always means short wavelength: gamma rays are a million-million times shorter than radio waves.

Every object constantly emits infrared radiation — you do, this desk does, and the hotter the object, the more infrared it radiates each second. That is the whole principle of a thermal camera. Which surfaces are best at it? The classic Leslie cube investigation fills a metal cube with boiling water so its four sides — matt black, matt white, polished metal, and a painted grey — sit at the same temperature. An infrared detector held the same distance from each face reads the radiation: matt black emits the most, shiny silver the least. Absorption tells the same story: shine an infrared lamp at two identical plates side by side (one matt black, one silver) with a thermometer behind each, and the black-backed plate warms faster.

Applications follow directly: houses are painted light colours in hot countries to reflect the Sun's infrared, firefighters' heat cameras spot warm bodies through smoke, and matt black panels sit behind solar-thermal water heaters to soak up the maximum radiation. A good exam line: good absorbers are good emitters — matt black does both jobs best, which is why it is the classic choice for radiator fins and the worst choice for summer clothing.