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 glycerine (which has the same refractive index ( = 1.5) as that of glass) then the equivalent focal length is F 2 . The ratio F 1 : F 2 will be :

1. A. 3 : 4
2. B. 2 : 1
3. C. 1 : 2
4. D. 2 : 3

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