For a curved mirror, the radius of curvature R is twice the focal length f:
[tex]R=2f[/tex] (1)
Since the diameter d is twice the radius R, we can rewrite (1) as
[tex] \frac{d}{2} = 2f [/tex]
From which we can calculate the focal length from the diameter:
[tex]f= \frac{d}{4}= \frac{28.0 cm}{4}=7.0 cm [/tex]
