SET UP AND SOLVE The center of curvature of the first surface is on the incoming side of the light, so R1 is negative: R1=−20cm. The center of curvature of the second surface is on the outgoing side of the light, so R2 is positive: R2=5.0cm. Then, 1ff===(n−1)(1R1−1R2)(1.52−1)(1−20cm−15cm)−7.7cm REFLECT This lens is a diverging lens—a negative lens—with a negative focal length. Part A – Practice Problem: A double-concave lens with index of refraction n=1.47 has two surfaces with radii of curvature with absolute values 12.5 cm and 10.5 cm . What is the focal length of the lens?