Question 1

Multiple choiceMedium1 mark

A monochromatic beam illuminates two identical slits, each of width aa whose centres are separated by dd The ratio d/ad/a is an integer greater than 1. Which order of the two-slit interference pattern is absent because it coincides with the first minimum of the single-slit diffraction envelope?

Show answer and explanation

Answer: B. At the first single-slit minimum, asin⁡θ=λa\sin\theta=\lambda A missing interference maximum occurs at the same angle, where dsin⁡θ=nλd\sin\theta=n\lambda Equating the angles gives n=d/an=d/a

A: reverses the ratio of slit width to centre separation.
C: uses the first diffraction-minimum index as the interference order without matching the angles.
D: uses twice the first-minimum condition, introducing an extra factor of 2.

Remember: an interference order is missing when its maximum coincides with a diffraction minimum.

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