Question

Because a concave lens cannot form a real image of a real object, it is difficult to measure its focal length precisely. One method uses a second, convex, lens to produce a virtual object for the concave lens. Under the proper conditions, the concave lens will form a real image of the virtual object! A student conducting a laboratory project on concave lenses makes the following observations: When a lamp is placed 42.7 cm to the left of a particular convex lens, a real (inverted) image is formed 36.6 cm to the right of the lens. The lamp and convex lens are kept in place while a concave lens is mounted 16.9 cm to the right of the convex lens. A real image of the lamp is now formed 34.3 cm to the right of the concave lens. What is the focal length of the convex lens?

Answer 1

focal length of the convex lens = 19.70cm

The concave lens is placed close to the convex lens, closer than the original image distance d _i 1 . This means that our image from the first lens is a virtual object for the concave lens, with an object distance of

focal length of the concave lens = -46.28cm