Question

how will you dertermine rough focal length of convex lens

Answer 1

We cannot use an optical method to determine the focal length of a convex mirror without using a convex (converging) lens since a convex mirror does not form a real image of an object in front of it or even parallel rays coming from infinity. Therefore we have to use a convex mirror to determine the focal length of the convex mirror by an indirect method.

Read more at: http://www.askphysics.com/how-to-find-the-focal-length-of-convex-mirror-using-convex-lens/
I hope it's helpful..