Find the diameter of the image of the moon in mm formed by a spherical concave mirror of focal length 7.6 m. The diameter of the moon is 1450 Km and the distance  between earth and moon is $3.8\times {10}^{5}km$  (mm).

The angle subtended by moon at pole of the mirror 
  $\theta =\frac{D}{s}=\frac{3450}{3.8\times {10}^5}=9.1\times {10}^{-3}rad$
 
The image of the moon is formed at the focal plane of the mirror (see figure). If d is the diameter of the image of the moon, then 
  $$\begin{gathered} \theta =\frac{d}{f} \\ d=\theta f=9.1\times {10}^{-3}\times 7.6=6.9\times {10}^{-2}m. \end{gathered}$$