Because a concave lens cannot form a real image of a real object, it is difficul
ID: 1407919 • Letter: B
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 40.7 cm to the left of a particular convex lens, a real (inverted) image is formed 35.1 cm to the right of the lens. The lamp and convex lens are kept in place while a concave lens is mounted 13.0 cm to the right of the convex lens. A real image of the lamp is now formed 32.9 cm to the right of the concave lens. What is the focal length of the convex lens?
What is the focal length of the concave lens?
Explanation / Answer
the image of the first lens becomes the object of the second lens
you will need a good diagram to keep track of distances
in this case the first image is 35.1-13.0 = 22.1 to the right of the concave lens
this is a virtual object so use -22.1
so 1/f = 1/(-22.1) + 1/32.9
F = -67.32 cm for the concave lens
For convex lens , 1/f = 1/40.7 + 1/35.1
F = 18.84 cm for convex lens