Question

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 $F1: F2$ will be:

• A. $2:1$
• B. $1:2$
• C. $2:3$
• D. $3:4$

Answer 1

Answer: (B)

$1:2$

Answer 2

The correct option is (B) :$1:2$