4. An equilateral prism deviates a ray of light through 23 for two angles of incidence, which differs by
23. The refractive index of prism is
5. The plane side of a planoconvex lens is silvered and the lens then acts like a concave mirror of 30cm focal length. The refractive index of lens is 1.5. What is the radius of curvature of this lens?