Two similar thin equi-convex lenses of focal length f each are kept coaxially in contact with each other, such that the focal length of the combination is F 1 . When the space between the two lenses is filled with glycerin (which has the same refractive index ( μ = 1.5 ) as that of glass), then the equivalent focal length is F 2 . The ratio F 1 : F 2 will be