Question

A convex lens of refractive index $15$ and focal length $18 \,cm$ in air is immersed in water. The change in focal length of the lens will be_______ cm (Given refractive index of water =$\frac{4}{3}$)

Answer 1

The correct answer is 54.
fH2​O​I​=(μH2​O​μg​​−1)(R2​)
=81​(R2​)
=(4fair​)1​
So, fH2​O​=4fair ​=72cm
So change in focal length =72−18=54cm