Question

you are provided with two lenses of focal length 20 cm and 40 cm respectively which lens you would you use to to obtain more convergent light​

Answer 1

I would use the lens with 20 cm focal length if it is a convex lens.

Answer 2

focal length of any point is the point where light converges.So if focal length is more then the lens has converged the light to a larger distance that it has converged less.Whereas if the focal length is less than the light is converged at a smaller distance that is converged more.
So,we can affirm that lens of 20 cm should be used to converge light to a larger extent.