The adjacent figure shows a thin plano-convex lens of refractive index `mu_1` and a thin plano-concave lens of refractive index `mu_2`, both having same radius of curvature R of their curved surfaces. The thin lens of refractive index ` mu_3` has radius of curvature R of both its surfaces. This lens is so placed in between the plano-convex and plano-concave lenses that the plane surfaces are parallel to each other.
The focal length of the combination is