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
(μ = 1.5) as that of glass) then the equivalent
focal length is ``F_2``. The ratio ``F_1:F_2``
will be :
(1) 2 : 1
(2) 1 : 2
(3) 2 : 3
(4) 3 : 4