A plano convex lens of refractive index ₁ and focal length ƒ₁ is kept in contact with another plano concave lens of refractive index ₂ and focal length ƒ₂. If the radius of curvature of their spherical faces is R each and ƒ₁ = 2ƒ₂, then ₁ and ₂ are related as -