Light that passes from a medium of one density into a medium of a different density is refracted, or bent. The degree of refraction depends on the comparative densities of the two media, as indicated by their refractive index. The refractive index of air is set at 1.00; the refractive index of the cornea, by comparison, is
1.38; and the refractive indices of the aqueous humor and lens are 1.33 and 1.40, respectively. Since the greatest difference in refractive index occurs at the air-cornea interface, the light is refracted most at the cornea.
The degree of refraction also depends on the curvature of the interface between two media. The curvature of the cornea is constant, but the curvature of the lens can be varied. The
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refractive properties of the lens can thus provide fine control for focusing light on the retina. As a result of light refraction, the image formed on the retina is upside down and right to left (fig. 10.30).
The visual field—which is the part of the external world projected onto the retina—is thus reversed in each eye. The cornea and lens focus the right part of the visual field on the left half of the retina of each eye, while the left half of the visual field is focused on the right half of each retina (fig. 10.31). The medial (or nasal) half-retina of the left eye therefore receives the same image as the lateral (or temporal) half-retina of the right eye. The nasal half-retina of the right eye receives the same image as the temporal half-retina of the left eye.
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