The eye is designed to focus light onto specialized nerve endings that translate the light waves and photons into pictures our brains can understand. Sight is the interpretation of those signals by the brain, which allows a person to interact and recognize the world around them.
Light Path Through the Eye
Light first enters the eye through the cornea. Behind the cornea is the lens, the refraction system of the eye. The lens focuses the light on the retina at the back of the eyeball like a camera, via refraction. Receptors lining the retina are activated by the specific intensity or wavelength of light hitting them, transmitting that information to the brain.
Anatomy of Eye Structures
The structure of the components of the eye are conducive to their place and function in the path of light through the eye. Most students learn about this important anatomical part by dissecting a cow eyeball in high school science class.
The cornea is a thin clear sheath covering the portion of the eye on the outside, or front, of the eyeball. Behind the cornea is a cavity filled with a nutrient-filled liquid called aqueous humor, which provides important nutrients and hydration to the cornea and lens. There are no blood vessels in the cornea because it must remain clear for the light to pass through uninterrupted. The lens is on the other side of the aqueous humor. The lens also has no blood vessels so that light can pass through unhindered.
The pupil, which we see as the black (or void) center of the eye, is the hole (an aperture) that controls how much light makes it from the cornea to the lens. The iris, or the colored part of the eye, is actually two sets of muscle fibers that control the size of the pupil. The lens is also adjusted by muscles called the ciliary body, which allow focusing for distance and movement.
Behind the lens is a chamber filled with a thick liquid called vitreous humor. This is the squishy part of the eye ball. Vitreous humor is semi-solid and helps the eye keep its shape; it also holds the retina in place. The retina is at the back of the inside of the eye and contains light receptors. The receptors are nerve endings with a direct link to the brain via the optic nerve.
Light Receptors in the Eye
The receptors in the eye are specialized nerve fibers called rods and cones. Rods detect light regardless of wavelength. These receptors help the brain determine light intensity - bright or dim. Cones detect, red, blue, or green wavelengths and are active in bright light. Each type of cone (red, green, or blue) detects that particular wavelength of light.
There is a blind spot at the back of the eye where the optic nerve passes through the eyeball and there are no receptors. Having two eyes makes up for this; it is called binocular vision. However, if you mark an X on a piece of paper, cover one eye, and slowly move the paper across your field of vision, the mark will disappear at some point because the light waves that bring it into your eye are falling on the blindspot.
Accessory structures protect the eye or make it function more efficiently. The flap of skin used to spread lubricant and cover the eyes when they are not in use, the eyelid opens and closes via various muscles and nerve impulses. The lubricant is secreted by tear ducts, called lacrimal glands, located along the nasal sinuses and under the eye (see illustration). The tears are a salt solution to keep the eye from drying out, and contains a lysozyme that protects the eye from bacteria. The conjunctiva is a thin mucous membrane lining the eye and eyelids. The eyelids also have hairs, or eyelashes, that aid in keeping out dust and harmful debris, as scratching the cornea would prevent light from passing through properly.