Question

In frog, the eyes are

• A. Myopic on land
• B. Hypermetropic in water
• C. Bulging
• D. All of the above

Answer 1

Answer: (D)

All of the above

The large round lens of the frog gives the animal a large field of view. The frog is naturally nearsighted (myopic) to -6 diopters giving it a focus of approximately 6 inches. Frogs and toads can change their focus by moving the lens out towards the cornea.

In frogs, the focus range is a few diopters and in toads, the focus range is 5 diopters giving the best myopia of -1 diopters. During this accommodation, the pupil also increases in size. The advantage of nearsightedness is that it blurs the background clutter making foreground object characterization much easier.

Eyes that are better adapted for terrestrial vision and are emmetropic (i.e., in focus) in the air tend to be hypermetropic (i.e., far-sighted) underwater, while eyes better adapted for aquatic vision and are emmetropic in water will tend to be myopic (i.e., near-sighted) in air. If the retinal image is to remain sharp in both media, the eye must cope with large changes in external refractive indices. The bulging eyes of most frogs allow them to see in front, to the sides, and partially behind them.