Practice portal › Wave Optics › Interference and Young's Double Slit Experiment
Asked in GSEB Board March 2016 · Intensity and coherent sources
Idea: coherence is whatever is needed for a steady interference pattern, so ask what keeps the fringes standing still.
At a point on the screen the two waves meet with a total phase difference made of a path part, (2π)/λ Δ x, and the initial phase difference of the two sources.
For the pattern to stay put, that total must not change with time.
The path part stays fixed only if both waves have the same frequency, so that λ is common to them.
The remaining part stays fixed only if the initial phase difference is constant.
So coherent sources have the same frequency and a constant initial phase difference.
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