Question

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

Options

• both concave

• both convex

• the mirror is concave and the lens is convex

• the mirror is convex, but the lens is concave

Answer 1

both 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.

Answer 2

 Both concave

According to the sign convention, the focal length for both a concave mirror and a concave lens is negative.