JAMB - Physics (1999 - No. 20)
The inside portion of part of a hollow metal sphere of diameter 20cm is polished. The portion will therefore form a
concave mirror of focal length 5cm
concave mirror of focal length 10cm
convex mirror of focal length 5cm
convex mirror of focal length 20cm
Forklaring
Since diameter of the sphere = 20cm, then, the radius of curvature = 10cm
Focal length, f = \(\frac{R}{2} = \frac{10}{2}\) = 5cm
When the inner/ inside portion of a sphere is polished/coated/painted, the outside becomes the reflecting part and the resulting mirror is called a convex mirror.
Thus, a convex mirror of focal length 5cm is formed.