Question
A spherical mirror and a thin spherical lens are each found to have a focal length of -15 cm. Based on the respective sign conventions, what are the likely types of the mirror and the lens?
Both convex
The mirror is concave and the lens is convex
Both concave
The mirror is convex and the lens is concave
Detailed Explanation
A focal length of -15 cm indicates a concave mirror (negative f by mirror convention) and, separately, a concave lens (negative f by lens convention) -- so both are concave.