Answer
10.5 cm
Work Step by Step
Here, we want to produce an image at infinity. The object must be at the focal point, as seen in Figure 23-14 with the rays reversed.
The focal length is half the radius of curvature.
$d_o=f=\frac{1}{2}r=\frac{1}{2}(21.0cm)=10.5cm$
