Draw diagrams of the following two lens examples to help answer the question We are taking the focal length of the first lens to be positive 

a). Imagine rays from infty being focused by the lens to point F1 A second lens with focal length f'20f is placed just behind the first lense. How is the focus of the first lens modified 

b). A pair of planoconvex lenses are arranged to expand a collimated laser beam by a factor of two If the first lens has a focal length of f, what is the focal length of the s.