Answer
0.4 m
Work Step by Step
Find the angular resolution using equation 25–7.
$$\theta=\frac{1.22\lambda}{D}$$
The smallest feature resolvable, L, is that angle multiplied by the distance to the surface, r, from the spy plane. (In other words, the ground feature subtends that angle.)
$$L=r \theta=\frac{1.22 r \lambda}{D}$$
$$D= \frac{1.22 r \lambda}{L}$$
$$1.22 \frac{(25000m)(580\times10^{-9}m)}{0.05m}$$
$$ = 0.4m$$
