# What is a wave in physics

## wave

wave, a spatially expanding excitation that transports energy. This allows waves to propagate as the Disruptions physical quantities, such as the deflection of particles in a medium or the field quantities of a physical field. The disturbance can be a one-off excitation or a periodic process. The speed of propagation of waves is always finite.

The wave propagation within a medium occurs through the excitation of particles to vibrate due to particles that are already vibrating. One speaks of Longitudinal waves or Longitudinal shaftswhen these vibrations occur in the direction of propagation of the wave. In the case of vibrations perpendicular to the direction of propagation, one speaks of Transverse waves or Longitudinal shafts. Water waves are an example of transverse waves, sound waves are longitudinal waves.

Excitations of physical fields, on the other hand, do not require a medium of propagation, so such waves can also propagate in a vacuum. The most important example are the electromagnetic waves, in which both the electrical and the magnetic field strength vibrate. Due to the form of Maxwell's equations, they can only appear as transverse waves. Light is also an electromagnetic wave, so that its propagation including diffraction, refraction and interference can be treated within the framework of wave theory (Huygens principle). The propagation of electromagnetic waves in matter can lead to very complicated conditions, in particular the investigation of electrical and magnetic waves in plasmas (plasma waves, extraordinary waves) is often only possible with a great deal of effort. In this case, the electromagnetic waves can be coupled with oscillations of the plasma particles, i.e. sound waves. The same applies to solids, in which phonons can be generated and destroyed through processes in which electromagnetic waves are involved. Electromagnetic waves always travel at the speed of light. The fact that the speed of propagation in solids does not correspond to the speed of light is an effect that can be explained by the constant absorption and time-delayed re-emission of the electromagnetic waves by the atoms of the medium.

As part of the Quantum theory the photons are assigned to the electromagnetic waves (quantum electrodynamics), which, due to their propagation at the speed of light, must have a rest mass of zero. Conversely, one can also assign particles with rest masses other than zero to matter fields and thus matter waves (De Broglie wave).

In the case of transverse waves, it can happen that the oscillations of the underlying physical quantity only occur in one spatial direction. In this case the wave is called linear polarized. In the case of matter waves, the polarization is given by the direction of the spin of the associated particle currents.

A surface within a wave, the points of which are in the same state of oscillation at the observed point in time (phase) are called Wave surface or Wavefront, the curves perpendicular to the wave fronts Wave normals. Depending on the shape of the wave fronts, one speaks of cylinder-, Bullet- or levels Waves. Plane waves result from the excitation of all points of a plane with the same phase, cylindrical waves with in-phase excitation along a straight line and spherical waves from excitation in one point in space.