Lainekiiruste

Lainekiiruste (Finnish for “wave speeds”) refers to the speeds at which wave phenomena propagate. In physics, the term covers measures such as phase velocity and group velocity and applies across acoustic, optical, and fluid-wave contexts.

Phase velocity is the speed at which a point of constant phase travels. It is defined as vp = ω/k, with ω the angular frequency and k the wavenumber. For a monochromatic wave, phase fronts move at vp.

Group velocity is the speed of the envelope of a wave packet, vg = dω/dk. It often indicates how energy or information propagates. In non-dispersive media, vg equals vp.

In dispersive media, different frequencies travel at different speeds, causing pulse spreading and distortion. The dispersion relation ω(k) determines how vp and vg vary with frequency. Understanding these relationships helps predict how complex signals evolve as they propagate.

Examples include light in materials where the refractive index depends on frequency, surface and deep-water waves where depth affects speed, and seismic waves in Earth’s interior. Dispersion relations are used to characterize media, infer material properties, and design optical, acoustic, and imaging systems.

Lainekiiruste is a core concept in physics and engineering, informing fields such as telecommunications, optics, seismology, and fluid dynamics.