The condition for **constructive interference** at a point in Young's double-slit experiment is that the path difference equals:
A$\lambda/4$
B$\lambda/2$
C$n\lambda$ (integer multiple of $\lambda$)
D$(2n+1)\lambda/4$
Answer & Solution
Correct answer: C. $n\lambda$ (integer multiple of $\lambda$)
Constructive: path difference = $n\lambda$ (waves in phase). Destructive: $(2n+1)\lambda/2$ (waves $\pi$ out of phase). $n$ = 0, ±1, ±2, … for successive bright fringes.
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