Coherence (physics)

Coherence in advanced physics is a phenomenon of electromagnetic waves.
In physics, two wave sources are perfectly coherent if they have a constant phase difference and the same frequency, and the same waveform. In such a case the waves plot as identical: their peaks and troughs occur at the same time, and they have the same amplitude.
Coherence is an ideal property of waves. It produces stationary (i.e. temporally and spatially constant) interference.
The idea has several distinct concepts. These are limiting cases which never quite occur in reality. However, they allow an understanding of the physics of waves, and have become an important concept in quantum physics.
More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets.