Two similar thin equi-convex lenses, of focal 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 lens is filled with glycerin (which has the same refractive index `(mu=1.5)` as that of glass) then the equivlent focal length is `F_(2)`, The ratio `F_(1): F_(2)` will be

Options

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

Answer

B

Explanation

Equivalent focal length in air `1/(F_1) = 1/f + 1/f = 2/f`

When glycerin is filled inside, glycerin lens behaves diverging lens of focal length (-f)
`1/(F_2) = 1/f + 1/f - 1/f = 1/f`
`(F_1)/(F_2) = 1/2`