Back-of-the-Envelope Physics by Clifford Swartz

Author:Clifford Swartz
Language: eng
Format: epub
Publisher: The Johns Hopkins University Press
Published: 2003-03-09T16:00:00+00:00

SEEING UNDER WATER

An early lesson in learning to swim is to open one’s eyes under water. It must be a little frightening for a child because one can’t see very well under water, even with eyes open. The lens of a healthy eye is a remarkable organ, canceling out imperfections due to spherical and chromatic aberrations. But the major focusing actually takes place at the boundary between air and cornea. The lens just makes second-order corrections to focus the light on the retina. When you are seeing under water, there is very little refraction as the light leaves the water and passes through the cornea. The index of refraction of the cornea and the liquid in the eye is very close to that of water.

Optometrists describe the focusing power of lenses in terms of diopters, , where f is the focal length measured in meters. The smaller the focal length of the lens, the more “powerfully” it focuses. A converging lens has positive power in diopters, and a diverging lens has a negative D. One advantage of this system is that for thin lenses that are close together, the power in diopters just adds linearly:

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