E2 The Eye as an Optical System: Refraction and Accommodation

Topic

Following the model of optical rays and interfaces, the cornea is the first surface a light ray encounters upon entering the eye; because the refractive index of air differs significantly from that of the corneal tissue itself, the air-cornea interface strongly refracts every ray passing through it. In fact, the cornea alone provides the majority of the eye's total optical power, precisely because no other interface within the eye involves as large a difference in refractive index as that found between air and the cornea.

After passing through the cornea, the light ray travels through the lens—the eye's second refractive structure—which contributes the remainder of the total optical power. Unlike the cornea, whose shape remains constant as light passes through it, the lens can alter its shape—and consequently its optical power—during the eye's operation; Gullstrand's schematic eye model simplifies both the cornea and the lens into a small number of idealized curved surfaces, each with a specific refractive index, allowing for precise calculation of where each ray is directed after traversing both structures.

When an object is distant, the light rays emanating from it reach the cornea nearly parallel to one another, and the fixed optical power provided by the cornea and the resting lens suffices to converge them precisely onto the retina. Conversely, when an object is close, the rays reaching the cornea are diverging, requiring greater optical power to converge them at that same retinal point; this additional power is supplied through accommodation—specifically, by altering the shape of the lens via the ciliary muscle, as described by the model of presbyopia. The total optical power of the eye at any given moment is, therefore, the sum of the fixed optical power provided by the cornea, the fixed optical power provided by the lens in its resting state, and the additional optical power provided by accommodation when viewing a nearby object. For a sharp image to form on the retina, this total optical power must be adjusted—at every moment—to the eye's axial length and the distance of the object being viewed, as described by the model of ametropia: if the total optical power fails to make the light rays converge exactly on the retina, the image forms either in front of or behind the retina, resulting in a blurred image.

For this reason, Gullstrand’s schematic eye model—which accurately describes how the cornea and lens refract light rays on their path to the retina, and how accommodation alters the lens's power based on object distance—serves as the foundation for all subsequent calculations regarding ocular refraction. Whether determining the optical prescription required for a specific eye, calculating the necessary power for an intraocular lens, or planning the extent of corneal reshaping in refractive surgery, any optical correction of the eye relies on this same model of the cornea and lens refracting light rays toward the retina.

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