Object inside f virtual image – Notice that when the object is inside the focal point the light rays never intersect, but from the viewer’s perspective they (the viewer is to the right of the lens in the diagrams appear to do so on the same side of the lens as the object above). Problems Relationship between focal length object distance (u) and image distance (v) (f), of a Lens* Power of a Lens = 1/focal length The unit of is m-1. Convention The power of a (convex) lens is taken as (+) {because is positive}. converging positive f The power of a (concave) lens is taken as (-) {because is negative}. diverging negative f Two Lenses in Contact
Object inside f – virtual image Notice that when the object is inside the focal point the light rays never intersect, but from the viewer’s perspective they appear to do so on the same side of the lens as the object (the viewer is to the right of the lens in the diagrams above). Problems Relationship between focal length (f), object distance (u) and image distance (v) of a Lens* Power of a Lens = 1/focal length The unit of is m-1. Convention The power of a converging (convex) lens is taken as positive (+) {because f is positive}. The power of a diverging (concave) lens is taken as negative (-) {because f is negative}. Two Lenses in Contact