If f (which is the distance from the focal point to the lens) is positive, then the "upstream" focus is on the side of the incoming light and the "downstream" focus is on the side of the ongoing light.
We wish to attempt to duplicate some of Newton's results with a lens and prism. A lens with focal length 10cm is placed in a ray of sunlight, and a prism is placed at the "downstream" focal point of the lens. The light passing through the prism is broken into its component colors. If it is an...