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In a Young's double-slit experiment the slit separation d = 0.3 mm and the screen distance D = 1 m. A parallel beam of wavelength 600 nm strikes the slits at an angle alpha. On the screen, point O is equidistant from the slits and PO = 11.0 mm. Which statements are correct? [JEE Advanced 2019] (1) For alpha = 0.36/pi degree there is destructive interference at O. (2) The fringe spacing depends on alpha. (3) For alpha = 0.36/pi degree there is destructive interference at P. (4) For alpha = 0 there is constructive interference at P.

1. (1) and (3)
2. (1), (2) and (3)
3. (2) and (4)
4. (1), (2), (3) and (4)

Correct answer: (1) and (3)

Solution

Tilting the incident beam adds a constant path difference d*sin(alpha) but does not change the fringe spacing (it only shifts the pattern). Evaluating the total path difference at O and at P for alpha = 0.36/pi degree shows half-integer multiples of lambda (destructive), while statement (2) is false and (4) is false.

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