1. The problem statement, all variables and given/known data A lens made of glass has one concave side with a radius of curvature of 5 cm, and one convex side with a raduis of curvature of 7.5 cm. What is the focal length of the lens? Is this a converging or diverging lens? 2. Relevant equations nglass=1.5 nair=1.0 (1/f)=(n-1)[(1/R1)+(1/R2) R1=5 cm R2=7.5 cm 3. The attempt at a solution 1/f=(1.5-1)[(1/5cm)+(1/7.5cm)] = .5(7.5cm+5cm/.375cm) = 16.667 So f=1/16.667=.06cm The answer is supposed to be -30 cm and diverging. I have done a lot of similar problems right, I don't understand what I keep doing wrong on this one.