Q. 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 :
A) 2 : 1
B) 1 : 2
C) 2 : 3
D) 3 : 4
Explanation
[Solution] $F_1 = f/2$. The glycerin forms a concave lens of focal length -f, so $1/F_2 = 2/f - 1/f$ and $F_2 = f$. Ratio 1 : 2. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.