A spherical mirror and a thin spherical lens have each a focal length of -15 cm. The mirror and the lens are likely to be
the mirror is concave and the lens is convex
the mirror is convex, but the lens is concave
By convention, the focal length of a concave mirror and a concave lens are taken as negative. Hence, both the spherical mirror and the thin spherical lens are concave in nature.
According to the sign convention, the focal length for both a concave mirror and a concave lens is negative.