Answer
1.8 degrees
Work Step by Step
Use equation 24–3a to calculate the angular width from the middle of the central peak to the first minimum. The width of the entire central peak is two times this angular width.
$$sin\theta_1=\frac{\lambda}{D}$$
$$\theta_1=sin^{-1}(\frac{\lambda}{D})$$
$$= sin^{-1}(\frac{680\times10^{-9}m}{0.0425\times10^{-3}m})=0.9168^{\circ}$$
$$\Delta \theta = 2 \theta_1=1.8^{\circ}$$
