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 \[(\mu =1.5)\] as that of glass) then the equivalent focal length is \[{{F}_{2}}\]. The ratio \[{{F}_{2}}\] : \[{{F}_{1}}\]will be: [NEET 5-5-2019]