Coherence (physics) facts for kids

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.

Images for kids

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  Figure 2: The amplitude of a wave whose phase drifts significantly in time τc as a function of time t (red) and a copy of the same wave delayed by 2τc(green). At any particular time t the wave can interfere perfectly with its delayed copy. But, since half the time the red and green waves are in phase and half the time out of phase, when averaged over t any interference disappears at this delay.

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  Figure 3: The amplitude of a wavepacket whose amplitude changes significantly in time τc (red) and a copy of the same wave delayed by 2τc(green) plotted as a function of time t. At any particular time the red and green waves are uncorrelated; one oscillates while the other is constant and so there will be no interference at this delay. Another way of looking at this is the wavepackets are not overlapped in time and so at any particular time there is only one nonzero field so no interference can occur.

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  Figure 4: The time-averaged intensity (blue) detected at the output of an interferometer plotted as a function of delay τ for the example waves in Figures 2 and 3. As the delay is changed by half a period, the interference switches between constructive and destructive. The black lines indicate the interference envelope, which gives the degree of coherence. Although the waves in Figures 2 and 3 have different time durations, they have the same coherence time.

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  Figure 11: Spectrally incoherent light interferes to form continuous light with a randomly varying phase and amplitude

See also

In Spanish: Luz coherente para niños