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 :

Question

( A ) $\pi \; radian$
( B ) $\frac{\pi}{4} radian$
( C ) $\frac{\pi}{8} radian$
( D ) $\frac{\pi}{2} radian$