a b c d

a

For Small angle $sin\frac{\theta}{2} = \frac{\theta}{2} = \frac{d}{2D}$ $\Rightarrow \theta = \frac{d}{D} = \frac{\lambda d}{\lambda D} = \frac{\lambda}{\beta}$ $\Rightarrow \beta = \frac{\lambda}{\theta}$