Wave Theory, Wavefronts & Doppler Effect question from JEE Main 2025 (2 Apr, Shift 1)

Answer: (b)

1. For the plane $x + y + z =$ constant, the normal (direction of propagation) is $\hat n = \dfrac{\hat i + \hat j + \hat k}{\sqrt3}$.
2. Angle with the $x$-axis: $\cos\alpha = \hat n \cdot \hat i = \dfrac{1}{\sqrt3}$.
3. So the answer is (b) $\cos^{-1}(1/\sqrt3)$.