Model of the eye, with near/far sight lenses, using arc lamp.

Model of the eye, with near/far sight lenses, using arc lamp. A glass globe with a lens, filled with water and Fluorescein, is a simple model for the eye. A 'normal' converging lens in the eye focuses at a point on the back of the eyeball. (I.E.: the image is focused at the fovea on the retina.) In a nearsighted eye, the lens focuses the image in front of the retina, and a corrective diverging lens is needed. In a farsighted eye, the lens focuses the image in back of the retina, and a corrective converging lens is needed. In this demonstration, the globe is 6.5" in diameter. The 'normal' lens has a f.l. of 3.5" in air, or 6.5" in water. The 'farsighted' lens has a f.l. = 5.5" in air or about 10" in water, and the corrective converging lens has a f.l. = +14" in air. The 'nearsighted' lens f.l. = 2" in air or about 4" in water, and the corrective diverging lens f.l. = -6" in air.