Q. In the set up shown in Fig the two slits, $S_{1}$ and $S_{2}$ are not equidistant from the slit $S$. The central fringe at $O$ is then
A) Always bright
B) Always dark
C) Either dark or bright depending on the position of $S$
D) Neither dark nor bright
Explanation
(c) If path difference $\Delta = \left( SS_{1} + S_{1}O \right) - \left( SS_{2} + S_{2}O \right) = n\lambda$
$n = 0,\ 1,\ 2,\ 3,\ldots$ the central fringe at $O$ is a bright fringe and if the path difference $\Delta = \left( n - \frac{1}{2} \right)\lambda,n = 1,2,3,\ldots$ the central bright fringe will be a dark fringe
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.