For a single slit diffraction pattern, the condition for the first minimum is $a \sin\theta = \lambda$, where $a$ is the width of the slit. This equation indicates that the path difference between wavelets from the two extreme edges of the slit is $\lambda$. The path difference between a wavelet from the top edge of the slit and a wavelet from the midpoint of the slit is $\frac{a}{2} \sin\theta = \frac{\lambda}{2}$. The corresponding phase difference $\phi$ is given by the relation $\phi = \frac{2\pi}{\lambda} \times \text{path difference}$. Therefore, $\phi = \frac{2\pi}{\lambda} \times \frac{\lambda}{2} = \pi$ radian.