Progressive Wave Equation question from NEET 1996 (AIPMT)

Answer: (a)

1. Path difference $\Delta x = \frac{\lambda}{2\pi}\,\Delta\phi$.
2. $\Delta\phi = 60^\circ = \frac{\pi}{3}$, so $\Delta x = \frac{\lambda}{2\pi}\cdot\frac{\pi}{3} = \frac{\lambda}{6}$
3. So the answer is (a) $\frac{\lambda}{6}$.