i) If f=0.5m for a glass lens, what is the powerof the lens? Ii) The radii of curvature of the faces of a double convex lens are 10 cm and 15 cm. Its focal length is 12 cm. What is the refractive index of glass? Iii) A convex lens has 20 cm focal length in air. What is focal length in water? (Refractive index of air-water =1.33, refractive index for air glass=1.5)