Chapter 23, Problem 65P

** You measure the focal length of a concave lens by first forming a real image of a light source using a convex lens. The image is formed on a screen 20 cm from the lens. You then place the concave lens halfway between the convex lens and the screen. To obtain a sharp image, you need to move the screen 15 cm farther away from the lenses. How does this experiment help you determine the focal length of the concave lens? What is the focal length?