Waves: what is actually oscillating
A wave is a disturbance that travels while transferring energy, without transporting matter along with it. Drop a stone in a pond and the water molecules mostly bob up and down — the *pattern* moves outward. Every wave description rests on three linked quantities:
- Wavelength (λ) — the distance of one complete cycle, measured in metres (m), millimetres, or — for light — nanometres (1 nm = 10⁻⁹ m).
- Frequency (f or ν) — the number of cycles passing a point each second, measured in hertz (Hz); 1 Hz = 1 s⁻¹.
- Period (T) — the time for one cycle, where T = 1/f. A 100 Hz tone has a period of 0.01 s.
They are welded together by the wave speed: v = f λ. Sound in air at 20 °C travels about 343 m/s, so a 1 kHz tone has a wavelength of roughly 0.34 m — about the length of a ruler. Visible light in vacuum spans roughly 400 nm (violet) to 700 nm (red), and because its speed is the familiar c ≈ 3 × 10⁸ m/s, a 500 nm green light oscillates about 6 × 10¹⁴ times per second.
Waves come in two mechanical flavours: transverse (the oscillation is perpendicular to travel — light, a vibrating string) and longitudinal (the oscillation is along the travel direction — sound, which is a pressure wave). Sound has no wavelength in vacuum because there is no medium to disturb; light needs no medium at all.
Two waves meeting can add (constructive interference, crest on crest) or cancel (destructive interference, crest on trough). That single fact explains microphone feedback, noise-cancelling headphones, and every diffraction pattern you will meet below.