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In a Young's double-slit experiment, a thin glass sheet is placed in front of the lower slit S2. The phase difference between the two interfering waves at the fifth minimum (counting minima from the centre) is:

1. 9 pi
2. 5 pi
3. 7 pi
4. 11 pi

Correct answer: 9 pi

Solution

Destructive interference (minima) occurs when the phase difference is an odd multiple of pi: phi = (2n - 1)pi, where n = 1 gives the first minimum. For the fifth minimum, n = 5, so phi = (2*5 - 1)pi = 9 pi.

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