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# For sustained interference, we need two sources that emit wave of

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## Sustained interference:An interference pattern constitutes constructive interference, which produces a maximum intensity, and destructive interference, which produces a minimum intensity.Sustained interference occurs when the interference pattern does not change with respect to time.The resultant intensity at any point is given as $I={{A}_{1}}^{2}+{{A}_{2}}^{2}+2{A}_{1}{A}_{2}\mathrm{cos}\varphi$, where ${A}_{1}$ and ${A}_{2}$ are the amplitude of the first and second wave respectively, and $\varphi$ is the phase difference between them.For sustained interference, the two waves must have equal amplitudes.Their phase difference $\varphi$ should also be constant.When the phase difference is not constant we do not get an interference pattern.Coherence is related to the definite phase relationship at different points in time and space. For a source to be coherent it must emit radiation of a single frequency or the frequency spread must be very small.This can be obtained by having light from the same source pass through two slits.The wavefront must have a shape that remains constant in time.Therefore, for sustained interference, the sources must be coherent.Hence, for sustained interference, we need two sources that emit waves of constant phase difference.

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