CBSE Class 12 · 2023 · Wave OpticsMultiple Choice

The fringe width is $\beta =\frac{\lambda D}{d}$. For a given setup $\lambda ,D,d$ are fixed, so every fringe has the same width — Assertion (A) is true.
Reason (R) correctly lists the quantities on which $\beta$ depends, so it is also true. However, the fact that $\beta$ depends on $\lambda ,D,d$ does not by itself explain why all fringes are equal; the equality follows because $\beta$ is independent of the fringe order $n$. Hence R is true but is not the correct explanation of A.