Question

According to the sign convention, which of the following is true for focal lengths of convex and concave lenses?

• A. Convex: Negative, Concave: Positive
• B. Convex: Positive, Concave: Negative
• C. Convex: Positive, Concave: Positive
• D. Convex: Negative, Concave: Negative

Answer 1

The correct option is B Convex: Positive, Concave: Negative

The parallel rays incident on a convex lens meet on the right hand side after converging. Hence, its focal length is positive.

The parallel rays incident on the concave lens appear to be meeting on the left hand side as shownin the figure below. Hence, its focal length is negative.