Chapter 25, Problem 55PP

To determine whether a frog can judge distance by means of the amount its lens must move to focus on an object, researchers covered one eye with an opaque material. An insect was placed in front of the frog, and the distance that the frog snapped its tongue out to catch the insect was measured with high-speed video. The experiment was repeated with a contact lens over the eye to determine whether the frog could correctly judge the distance under these conditions. If such an experiment is performed twice, once with a lens of power −9 diopters and once with a lens of power −15 diopters, in which case does the frog have to focus at a shorter distance, and why?

1. A.    With the −9-diopter lens; because the lenses are diverging, the lens with the longer focal length creates an image that is closer to the frog.
2. B.     With the −15-diopter lens; because the lenses are diverging, the lens with the shorter focal length creates an image that is closer to the frog.
3. C.     With the −9-diopter lens; because the lenses are converging, the lens with the longer focal length creates a larger real image.
4. D.    With the −15-diopter lens; because the lenses are converging, the lens with the shorter focal length creates a larger real image.