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_1 :F_2` will be :

Options

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

Answer

B, C

Explanation

`(1)/(F_1)=1/f+1/f=2/f`
`therefore F_1=f/2`
and `F_2=f` given
`therefore(F_1)/(F_2)=1/2`