1. The problem statement, all variables and given/known data Determine the focal length of a plano-concave lens (n=1.5) which has a radius of curvature of 10 cm. What is its power in diopters? 2. Relevant equations None given. 3. The attempt at a solution 1/f=(n-1)(1/R_1-1/R_2)=(1/2)(-0.1)=-1/20. Thus f=5 cm and P=-20 m^(-1). Is this right?