Download the app


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 F1. When the space between the two lenses is filled with glycerin (which has the same refractive index (μ=1.5) as that of glass) then the equivalent focal length is F2. The ratio F1:F2 will be

Remember concepts with our Masterclasses.

80k Users
60 mins Expert Faculty Ask Questions
3 : 4
2 : 1
1 : 2
2 : 3

Ready to Test Your Skills?

Check Your Performance Today with our Free Mock Tests used by Toppers!

detailed solution

Correct option is C

According to lens maker's formula  1f=(μ-1)1R1-1R2 1f=(μ-1)1R-1-R=(1.5-1)2R=1RTwo similar equi-convex lenses of focal length f each are held in contact with each other.The focal length F1 of the combination is given by1F1=1f+1f=2f; F1=f2=R2                   .........(i)For glycerin in between lenses, there are three lenses, one concave and two convexFocal length of the curve lens is given 1f'=(1.5-1)-2R=-1RNow, equivalent focal length of the combination is given by1F2=1f+1f'+1f;1F2=1R-1R+1R=1RF2=R                                                                      ..........(ii)  Dividing equation (i) by (ii), we get F1F2=12

Talk to our academic expert!


Are you a Sri Chaitanya student?


Create Your Own Test
Your Topic, Your Difficulty, Your Pace

Similar Questions

Image of the sun is formed by a binconvex lens of focal length f. The image is a circular patch of radius r and is formed on the focal plane of the lens. Choose the correct statement from the following 

phone icon
whats app icon