Waves are disturbances that propagate through space, transferring energy without moving matter. They are classified as transverse (perpendicular oscillation) or longitudinal (parallel oscillation) based on the direction of particle motion relative to wave propagation. All waves share common properties including wavelength, frequency, period, and amplitude.
Observe water waves in a ripple tank or demo, draw particle motion diagrams, compare spring waves with sound waves. Hands-on visualization of wave motion clarifies the distinction between particle motion and wave propagation.
A wave is not a moving thing — it is a moving pattern. Imagine a crowd doing a stadium wave: no person actually travels around the stadium; each person simply rises and sits at the right moment. The disturbance propagates, but the participants stay roughly in place. This is the central insight of wave physics: waves transfer energy through a medium (or through empty space) without transporting the medium itself. A water wave moves across the ocean's surface, but each water molecule just traces a small circle, returning nearly to where it started.
Waves are classified by the geometric relationship between how the medium oscillates and which direction the wave travels. In a transverse wave, the particles oscillate perpendicular to the direction of wave propagation. Imagine shaking one end of a horizontal rope up and down — the rope wiggles up and down, but the wave pattern moves horizontally along the rope. Light and all electromagnetic waves are transverse. In a longitudinal wave, particles oscillate parallel to the direction of travel — they compress together and then spread apart in the same direction the wave is moving. Sound in air is longitudinal: the air molecules bunch up (compression) and spread out (rarefaction) along the direction the sound travels. A Slinky stretched along the floor demonstrates longitudinal waves clearly: squeeze a few coils together and release them, and the compression pulse travels to the far end.
All waves share four measurable properties. Wavelength (λ) is the physical distance between consecutive identical points on the wave — for example, crest to crest. Frequency (f) is how many complete oscillation cycles pass a fixed point per second, measured in hertz (Hz). Period (T) is the time for one complete cycle: T = 1/f. Amplitude is the maximum displacement from the undisturbed equilibrium — the height of the crest above the resting level. Amplitude carries information about the wave's energy; a higher amplitude means more energy transported (energy is proportional to amplitude squared). Wave speed v = fλ connects these properties: a wave with a 2 Hz frequency and a 3 meter wavelength travels at 6 m/s. The speed itself is determined by the medium, not by the wave source — sound in air is always ~343 m/s at room temperature regardless of what is making the sound.
One important boundary case: electromagnetic waves — light, radio waves, X-rays, microwaves — are transverse waves that require no medium at all. They propagate through the vacuum of space because they are oscillating electric and magnetic fields that sustain each other. Every other wave type discussed in introductory physics (sound, water waves, seismic waves, waves on strings) requires a physical medium. This distinction matters because it explains how sunlight reaches Earth across 150 million kilometers of near-vacuum, while sound cannot — there is no material for the pressure oscillations to travel through in space.
This is a foundational topic with no prerequisites.
No prerequisites — this is a starting point.