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

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