Interference



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Two sources of light are said to be coherent if the waves emitted from them have the same frequency and are 'phase-linked'; that is, they have a zero or constant phase difference.

                       To obtain an expression for the light intensity at point P, the intensity of a wave is proportional to the square of the resultant electric field magnitude at that point, we can therefore express the light intensity at point P as:

Most light-detecting instruments measure time-averaged light intensity, and the time-averaged value of over one cycle is ½ therefore, we can write the average light intensity at point P as