Traveling Disturbances
A wave is a disturbance that travels through space, carrying energy without transporting matter. Waves are built from oscillations (Lesson 4): each point of the medium oscillates in place while the disturbance propagates. Water waves, sound, light, and vibrations on a string are all waves, described by the same mathematics.
Key quantities:
- Wavelength — the distance between successive repeats (crest to crest).
- Frequency — oscillations per second (hertz); the period is .
- Amplitude — the maximum displacement, related to the wave's energy.
- Speed — how fast the disturbance travels, tied to the others by the fundamental wave relation:
Waves come in two types by the direction of oscillation: transverse (medium oscillates perpendicular to travel — light, a plucked string) and longitudinal (oscillation along the travel direction — sound).
The signature behavior that distinguishes waves from particles is superposition and interference: when two waves overlap, their displacements add. Where crests align, they reinforce — constructive interference (larger amplitude); where a crest meets a trough, they cancel — destructive interference. This produces standing waves, beats, and the interference patterns that reveal the wave nature of light. Diffraction (bending around obstacles) and the Doppler effect (frequency shift from relative motion) are further wave hallmarks.
A crucial conceptual point already noted: a wave carries energy, not matter. A cork bobs up and down as water waves pass but does not travel with them; the medium oscillates locally while the wave — and its energy — moves onward. This distinguishes a wave from a stream of moving particles.
Common pitfall: thinking a wave carries matter along with it, and confusing frequency with period. A wave transports energy — the medium oscillates in place (a cork bobs without drifting). And frequency and period are reciprocals (), not the same: high frequency means short period, so a large frequency pairs with a small period, not a large one.