A diffraction grating (in air or vacuum), is illuminated by coherent (laser) lig

Question

A diffraction grating (in air or vacuum), is illuminated by coherent (laser) light with wavelength lambda and wave oscillation period T = lambda/c. The 5th order intensity maximum, to the right of the central intensity maximum M, is located at point Q, as shown in the figure below. A wave crest A, from slit R, and a wave crest B, from the neighboring slit S to the right of R, have departed at the same time from their respective slits of origin. Therefore, at Q, B will arrive 6 periods before A. B will arrive 9/2 periods before A. B will arrive 5 periods before A. A will arrive 5 periods before B. A and B will arrive at the same time.

Explanation / Answer

Since there is a maxima at Q, the path difference must be integral multiples of wavelength. This is to say that the wave periods from different slits will be integral multiples of each other.

Now, B will have to cover less distance in the same period than A and so B will arrive 5 periods before A [option C].

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