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Asked in GSEB Board March 2022 · Single slit diffraction
Idea: compare the two patterns fringe by fringe — interference fringes are uniform, diffraction fringes are not.
In two-slit interference the maxima are equally spaced, (λ D)/d apart, and all of the same intensity, so the statements about interference hold.
In single-slit diffraction the minima fall at a sin θ = nλ, so the central maximum runs from -λ/a to +λ/a in sin θ and is twice as wide as each secondary maximum after it.
The secondary maxima also fade quickly away from the centre, so a diffraction pattern is uneven in width as well as in brightness — which is what makes the first statement, about the central fringe, true.
So the false statement is that diffraction fringes are of equal thickness.
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