A narrow slit S is transmitting light of wavelength $λ$ and it placed at a distance d above a large plane mirror is shown in figure.
The light coming directly from the slit and the coming after reflection interfere at a screen placed at a distance D $(D ≫ d)$ from the slit. What will be the intensity at a point just above the mirror at point O?

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answer is A.

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Detailed Solution

Just above the point O, direct waves from source and reflected waves have a phase different $π$ (due to reflection from mirror). So these waves interfere destructively due to which a dark fringe is obtained at O. Hence intensity of light at O will be zero.

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