### NEET Solved Paper 2019 Question 2

##### 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 $ F _2 $ : $ F _1 $ will be: [NEET 5-5-2019]

#### Options:

A) 2 : 3

B) 3 : 4

C) 2 : 1

D) 1 : 2

## Show Answer

#### Answer:

Correct Answer: D

#### Solution:

$ \frac{1}{F _1}=\frac{1}{f}+\frac{1}{f} $

$ F _1=f/2 $

$ \frac{1}{F _1}=\frac{1}{f}( -\frac{1}{f} )+\frac{1}{f} $

$ \frac{1}{F _2}=\frac{1}{f} $

$ F _2=f $

$ F _1:F _2=\frac{f}{2}:f=\frac{1}{2}:1=1:2 $