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…

Two similar thin equi-convex lenses, of focal length $\displaystyle f$ each, are kept coaxially in contact with each other such that the focal length of the combination is $\displaystyle \mathrm{F}_1$. When the space between the two lenses is filled with glycerin (which has the same refractive index ( $\displaystyle \mu=1.5$ ) as that of glass) then the equivalent focal length is $\displaystyle \mathrm{F}_2$. The ratio $\displaystyle \mathrm{F}_1: \mathrm{F}_2$ will be :