Eye: Structure and Optics

Structure of the Eye

The eye in adult humans is approximately a sphere about 24 millimeters (mm) in diameter. Externally, it is covered by a resistant and flexible tissue called sclera, except in the anterior part where it turns to the transparent cornea that allows the light to pass into the eye. Within the sclera are two other layers: the choroid, which is a mesh of blood vessels to provide nutrients, and the retina, where the light is absorbed by the photoreceptors as the second step of the visual process. The eye can be moved by the action of six external muscles that permit fixation and the scanning of the visual environment.

The light reaching the eye is first transmitted and refracted by the cornea. This is a thin transparent layer free of blood vessels of about 12 mm in diameter with slightly more than 0.6 mm thickness in the central part. An aqueous tear film on the cornea ensures that the first optical surface is smooth to provide the best image quality. After the cornea, there is a small space, called the anterior chamber, that is filled with a water-like substance, the aqueous humor. In the posterior area of this space is the iris, a muscle with a central hole the size of which depends on the iris's contraction. The iris is a diaphragm with characteristic color, depending on the amount and distribution of pigments. The pupil is the variable opening in the center of the iris that limits the amount of light passing into the eye. The pupil size changes with the ambient light, from less than 2 mm in diameter in bright light to more than 8 mm in the dark. Other factors, such as emotions or certain drugs, also affect the size of the pupil. The pupil controls retinal illumination and limits the rays entering the eye to improve the images.

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