How the Human Eye Works

The human eye functions like a camera.
The human eye functions like a camera. Tim Flach / Getty Images

Members of the animal kingdom use different strategies to detect light and focus it to form images. Human eyes are "camera-type eyes," which means they work like camera lenses focusing light onto film. The cornea and lens of the eye are analogous to the camera lens, while the retina of the eye is like the film.

Eye Structure and Function

Parts of the human eye.
Parts of the human eye. RUSSELLTATEdotCOM / Getty Images

To understand how the eye sees, it helps to know the eye structures and functions:

Cornea: Light enters through the cornea, the transparent outer covering of the eye. The eyeball is rounded, so the cornea acts as a lens. It bends or refracts light.

Aqueous Humor: The fluid beneath the cornea has a composition similar to that of blood plasma. The aqueous humor helps to shape the cornea and provides nourishment to the eye.

Iris and Pupil: Light passes through the cornea and aqueous humor through an opening called the pupil. The size of the pupil is determined by the iris, the contractile ring that is associated with eye color. As the pupil dilates (gets bigger), more light enters the eye.

Lens: While most of the focusing of light is done by the cornea, the lens allows the eye to focus on either near or distant objects. Ciliary muscles surround the lens, relaxing to flatten it to image distant objects and contracting to thicken the lens to image close-up objects.

Vitreous Humor: A certain distance is required to focus light. The vitreous humor is a transparent watery gel that supports the eye and allows for this distance.

The Retina and the Optic Nerve

Diagram of the structure of the retina surface: The brown band at top consists of optic nerve. The purple structures are rods, while the green structures are cones.
Diagram of the structure of the retina surface: The brown band at top consists of optic nerve. The purple structures are rods, while the green structures are cones. Spencer Sutton / Getty Images

The coating on the interior back of the eye is called the retina. When light strikes the retina, two types of cells are activated. Rods detect light and dark and help form images under dim conditions. Cones are responsible for color vision. The three types of cones are called red, green, and blue, but each actually detects a range of wavelengths and not these specific colors. When you focus clearly on an object, light strikes a region called the fovea. The fovea is packed with cones and allows sharp vision. Rods outside the fovea are largely responsible for peripheral vision.

Rods and cones convert light into an electric signal that is carried from the optic nerve to the brain. The brain translates nerve impulses to form an image. Three-dimensional information comes from comparing the differences between the images formed by each eye.

Common Vision Problems

In myopia or near-sightedness, the cornea is overly curved. The image focuses before light strikes the retina.
In myopia or near-sightedness, the cornea is overly curved. The image focuses before light strikes the retina. RUSSELLTATEdotCOM / Getty Images

The most common vision problems are myopia (nearsightedness), hyperopia (far sightedness), presbyopia (age-related farsightedness), and astigmatism. Astigmatism results when the curvature of the eye isn't truly spherical, so light is focused unevenly. Myopia and hyperopia occur when the eye is too narrow or too wide to focus light onto the retina. In nearsightedness, the focal point is before the retina; in farsightedness it is past the retina. In presbyopia, the lens is stiffened so it's hard to bring close objects into focus.

Other eye problems include glaucoma (increased fluid pressure, which can damage the optic nerve), cataracts (clouding and hardening of the lens), and macular degeneration (degeneration of the retina).

Weird Eye Facts

Many insects see ultraviolet light.
Many insects see ultraviolet light. I love nature / Getty Images

The functioning of the eye is fairly simple, but there are some details you might not know:

  • The eye acts exactly like a camera in the sense that the image formed on the retina is inverted (upside down). When the brain translates the image, it automatically flips it. If you wear special goggles that make you view everything upside down, after a few days your brain will adapt, again showing you the "correct" view.
  • People don't see ultraviolet light, but the human retina can detect it. The lens absorbs it before it can reach the retina. The reason humans evolved to not see UV light is because it has enough energy to damage the rods and cones. Insects do perceive ultraviolet light, but their compound eyes don't focus as sharply as human eyes, so the energy is spread out over a larger area.
  • Blind people who still have eyes can sense the difference between light and dark. There are special cells in the eyes that detect light, but aren't involved in forming images.
  • Each eye has a small blind spot. This is the point where the optic nerve attaches to the eyeball. The hole in vision isn't noticeable because each eye fills in the other's blind spot.
  • Doctors are unable to transplant an entire eye. The reason is that it's too hard to reconnect the million-plus nerve fibers of the optic nerve.
  • Babies are born with full-size eyes. Human eyes stay about the same size from birth until death.
  • Blue eyes contain no blue pigment. The color is a result of Rayleigh scattering, which is also responsible for the blue color of the sky.
  • Eye color can change over time, mainly due to hormonal changes or chemical reactions in the body.

References

  • Bito, LZ; Matheny, A; Cruickshanks, KJ; Nondahl, DM; Carino, OB (1997). "Eye Color Changes Past Early Childhood". Archives of ophthalmology115 (5): 659–63. 
  • Goldsmith, T. H. (1990). "Optimization, Constraint, and History in the Evolution of Eyes". The Quarterly Review of Biology65(3): 281–322.
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Helmenstine, Anne Marie, Ph.D. "How the Human Eye Works." ThoughtCo, Dec. 6, 2017, thoughtco.com/how-the-human-eye-works-4155646. Helmenstine, Anne Marie, Ph.D. (2017, December 6). How the Human Eye Works. Retrieved from https://www.thoughtco.com/how-the-human-eye-works-4155646 Helmenstine, Anne Marie, Ph.D. "How the Human Eye Works." ThoughtCo. https://www.thoughtco.com/how-the-human-eye-works-4155646 (accessed May 22, 2018).