how the eye works Archives - Medical Information Illustrated

Anatomy of the eye

Below is a narrated animation of eye anatomy and common defects. Click here to license this video (and other related videos) on Alila Medical Media website.

The eyeball is roughly a sphere of about one inch in diameter. The main components of the eye include:

– The cornea – the transparent front part of the eye. The cornea refracts light and accounts for about two-thirds of the eye’s total focusing power.

– The iris – the pigmented part of the eye that makes up the eye color. The iris regulates the amount of light that enters the eye by adjusting the size of the pupil – an opening in the center of the iris.

– The crystalline lens – a clear biconvex structure located behind the pupil and helps to focus light further. The lens is capable of changing its shape to accommodate near vision.

Fig. 1 : Anatomy of human eye. Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

Light refracted by the cornea and the lens creates an image of the visual object on the retina. The retina is a light-sensitive tissue lining the inner surface of the eye. Within the retina, optical information is converted into neural action potentials which are then transmitted to the visual cortex of the brain through the optic nerve.

– The fovea (fovea centralis) is the (small) central area of the retina where the sharpest central vision is achievable.

Common eye defects

In the normal eye, light rays converge right on the retina. This results in sharp vision.

Fig. 2 : Light focusing in normal vision (upper panel), hyperopia (middle panel) and myopia (lower panel). Click on image to see a larger version on Alila Medical Media website where the image is also available for licensing.

In myopia, or nearsightedness, a condition in which the eyeball is too long, light rays converge before they reach the retina. The focal plane is located in front of the retina resulting in blurry vision. This happens when the person is looking at faraway objects. Myopia is corrected with concave lenses which diverge the light rays slightly before they enter the eye (Fig. 2).

In hyperopia, or farsightedness, a condition in which the eyeball is too short, light rays have not yet converged when they reach the retina. The focal plane is located behind the retina resulting in blurry vision. This happens when the person is looking at nearby objects. Hyperopia is corrected with convex lenses which converge the light rays slightly before they enter the eye (Fig.2).