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Thermodynamics

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Coherent Sources

Two sources are said to be coherent if they emit light waves of the same frequency, nearly the same amplitude and are always in phase with each other. It means that the two sources must emit radiations of the same colour (wavelength). In actual practice it is not possible to have two independent sources which are coherent. But for experimental purposes, two virtual sources formed from a single source can act as coherent sources. Methods have been devised where (i) interference of light takes place between the waves from the real source and a virtual source (ii) interference of light takes place between waves from two virtual sources formed due to a single source. In all such cases, the two sources will act, as if they are perfectly similar in all respects.

Since the wavelength of light wave is extremely small (of the order of 10^-8 cm), the two sources must be narrow and must also be close to each other. Maximum intensity is observed at a point where the phase difference the two waves reaching the point is a whole number multiple of wavelength. For minimum intensity at a point, the phase difference between the two waves reaching the point should be an odd number multiple of π or the path difference between the two waves should be an odd number multiple of half wavelength.