Answer: Horse SenseThe ciliary muscles in a horse’s eye

Chapter , Problem 7P

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Problem 7P

Horse Sense

The ciliary muscles in a horse’s eye can make only small changes to the shape of the lens, so a horse can’t change the shape of the lens to focus on objects at different distances as humans do. Instead, a horse relies on the fact that its eyes aren’t spherical. As Figure V.1 shows, different points at the back of the eye are at somewhat different distances from the front of the eye. We say that the eye has a “ramped retina”; images that form on the top of the retina are farther from the cornea and lens than those that form at lower positions. The horse uses this ramped retina to focus on objects at different distances, tipping its head so that light from an object forms an image at a vertical location on the retina that is at the correct distance for sharp focus.

A horse is looking straight ahead at a person who is standing quite close. The image of the person spans much of the vertical extent of the retina. What can we say about the image on the retina?

A. The person’s head is in focus; the feet are out of focus.

B. The person’s feet are in focus; the head is out of focus.

C. The person’s head and feet are both in focus.

D. The person’s head and feet are both out of focus.