[" 4.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)" the focal "],[" space between the two lenses is "F_(1)" .When the "],[" glyce between the two lenses is filled with "],[" glycerin (which has that of glass) then the "],[" index "(mu=1.5)" as that of glass) then the "],[" equivalent focal length is "F_(2)" .The ratio "],[F_(1):F_(2)" will be "],[[" (a) "3:4," (b) "2:1],[," (NEET "2019)]]