Question
Why is the interference pattern not detected, when two coherent sources are far apart?
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Answer
Fringe width of interference fringes, is given by $\beta=\frac{\text{D}\lambda}{\text{d}}\alpha\frac{1}{\text{d}},$.If the sources are far apart; d is large; so fringe width $(\beta)$ will be so small that the fringes are not resolved and they do not appear separate. That is why the interference pattern is not detected for large separation of coherent sources.
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