Focal length question
A converging glass lens (index of refraction 1.63) has a focal length of 40 cm in air. Find its focal length when it is immersed in a liquid, which has an index of refraction 1.32.
The way I set it up was...
The original equation: 1/f=(n-1)(1/r1-1/r2)
The (1/r1-1/r2) part of the equation stays constant so to "manipulate" the equation to solve for f2 when the lens is in the water...
f(liq)/f(lens) = (nlens-1)/(nliq-1)
x/40 cm = (1-1.63/(1-1.32)
I got the question wrong so I was wondering how to solve this problem correctly?