Given, Radius of curvature of the convex lens is R and μ=1.5
When lens is placed on a layer of water then focal length of the combination is f. Thus the first measurement gives the focal length of combinafioni.e. f =x.
![](https://www.sarthaks.com/?qa=blob&qa_blobid=8788901008451863052)
In the second measurement, we get the focal length f1 of convex lens i.e. f1=y, therefore the focal length of Plano connex lens of water f2 is given as
![](https://www.sarthaks.com/?qa=blob&qa_blobid=13653966578826563744)
![](https://www.sarthaks.com/?qa=blob&qa_blobid=2302114284674325225)
Now again for the plano-convex lens of air, if image coincide at 2f1 then for case II we have
![](https://www.sarthaks.com/?qa=blob&qa_blobid=16375510169503719499)
This is the required expression for the refractive index of the liquid.