31. At the first minimum adjacent to the central maximum of a single-slit diffraction pattern, the phase difference between the Huygen’s wavelet from the edge of the slit and the wavelet from the midpoint of the slit is :

31. At the first minimum adjacent to the central maximum of a single-slit diffraction pattern, the phase difference between the Huygen’s wavelet from the edge of the slit and the wavelet from the midpoint of the slit is :

(a) \(\frac{\pi}{2}\) radian

(b) \(\pi\) radian

(c) \(\frac{\pi}{8}\) radian

(d) \(\frac{\pi}{4}\) radian

Answer :  (b) \(\pi\) radian

\( \text{1st dark fringe}\)

\( \text{P.d} = 1 \frac{\lambda}{2}\)

\( \text{Phase difference} = \frac{2\pi}{2} = \pi \)