Solve this:

Q(i). If f = 0.5 m for a glass lens, what is the power of 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 = $\frac{4}{3}$, refractive index for air-glass = $\frac{3}{2}$,)