A concave lens of glass, refractive index 1.5, has both surfaces of same radius of curvature R. On immersion in a medium of refractive index 1.75, it will behave as a

fIfa= aμg−1 Iμg−1=1.5−11.51.75−1=1.75×0.500.25=−3.5∴fR=−3.5fa⇒fI=+3.5R∵fa=RHence on immersing the lens in the liquid, it behaves as a converging lens of focal length 3.5 R.