A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different materials refractive indices `mu_(1)` and `mu_(2)` and R is the radius curvature of the curved surface of the lenses, the focal length of the combination is

A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different material of refractive indices mu_(1) and mu_(2) and R is the radius of curvature of the curved surface of the lenses, then focal length of the combination is

A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different material of refractive indices mu_(1) and mu_(2) and R is the radius of curvature of the curved surface of the lenses, then focal length of the combination is

A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different material of refractive indices mu_(1) and mu_(2) and R is the radius of curvature of the curved surface of the lenses, then focal length of the combination is

A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive index mu_(1)=4//3 and mu_(2)=6//5 and R = 40 cm is the radius of curvature of the curved surface of the lenses, then the focal length of combination (in meters) is.