Sec 4 Combined Science (Physics) Wave Properties notes

Flick a stretched rope and a hump travels down it, but no piece of rope reaches the far end — each piece simply moves up and down about its own rest position. Push-and-shake a slinky spring and a pulse runs along it. In a ripple tank, a floating cork just bobs up and down as ripples pass. In every case the wave carries energy along, without transferring matter with it. That statement — energy yes, matter no — is the headline of the whole topic.

Draw the crests of continuous ripples and you have drawn wavefronts: lines joining points that are in step (in phase) — usually drawn through crests. The distance between one wavefront and the next is one wavelength. Wavefronts make wave behaviour easy to sketch: straight lines for flat ripples, circles for ripples from a dipped finger.

Amplitude is the maximum displacement of a point from its rest position — the height of a crest above the flat line (never the whole crest-to-trough height, which is twice the amplitude). Wavelength (λ, metres) is the distance between two successive points in step — crest to next crest, or compression to next compression. Frequency (f, hertz) counts waves passing a point per second. Period (T, seconds) is the time for one complete wave to pass, and it is the reciprocal: T = 1 ÷ f. Speed (v, m/s) is how fast the wave pattern travels.

Graphs carry these numbers: a displacement–time graph at one point gives the amplitude (peak height) and the period (time between successive crests); a displacement–distance graph frozen in time gives the amplitude and the wavelength (distance between successive crests). Check the axis label first — the same-shaped graph means different things with time or distance on the x-axis.

One equation ties the wave numbers together: speed = frequency × wavelength, v = fλ. Sense-check it: more waves per second (higher f), each of longer length (bigger λ), means the pattern sweeps along faster. Keep units matched — Hz and metres give m/s.

Transverse waves vibrate perpendicular (at right angles) to the direction of travel: flick a rope and it moves up-down while the wave runs along horizontally. Water ripples and light (all electromagnetic waves) are transverse. Longitudinal waves vibrate parallel to the direction of travel: push-shake a slinky and compressions race along it. Sound is the classic longitudinal wave.

In a longitudinal wave the particles bunch together in compressions (high density, high pressure regions) and spread apart in rarefactions (low density, low pressure), and this pattern travels onward — the particles themselves only shuffle back and forth about fixed positions.

Every sound begins with a vibrating source: a guitar string, a drum skin, your vocal cords, a tuning fork. The vibrating surface pushes and pulls the air around it, piling particles into compressions and thinning them into rarefactions — a longitudinal wave marching outward at about 340 m/s in air.

Sound needs a medium: no particles, no sound. A bell in a jar falls silent as the air is pumped out — and in the vacuum of space, explosions are silent. Solids and liquids carry sound too (often faster than air): hear a train through the rail before you hear it through the air.

Two knobs control what a sound is like. Loudness tracks the amplitude: hit the drum harder, bigger vibrations, taller wave, louder sound. Pitch tracks the frequency: tighter, shorter strings vibrate faster — higher frequency, higher pitch (a mouse squeaks at high f, a foghorn booms at low f).

Bounce sound off a hard wall and you hear an echo — the reflected wave. Because the sound travels to the surface and back, the distance to it is d = v × t ÷ 2, half the total path. Ships time echo pulses off the seabed to map depth; bats and blind people sense their surroundings the same way with much shorter times.