A plane-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 indices \(\mu_1\) and \(\mu_2\) and \(R\) is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is:
1. \(\frac{R}{2(\mu_1+\mu_2)}\)
2. \(\frac{R}{2(\mu_1-\mu_2)}\)
3. \(\frac{R}{(\mu_1-\mu_2)}\)
4. \(\frac{2R}{(\mu_2-\mu_1)}\)