Eyes

Globular organs in the head through which people and animals see.

Sight is the most precious of our five senses — sight, hearing, smell, taste, and touch. We rely on this sense more than any other. The human eye is the organ through which we see. We use our eyes in almost everything we do. If you are curious about your eyes then this is the place for you!

The human eye is the second most complex organ in our body after the brain. Although it is only the size of a ping pong ball, your eye has 2,000,000 working parts! To understand how the eye works, we'll first look at the main parts. This is called the anatomy of the eye.

• The eyeball is a spherical structure, about 2.5 cm (1 in) in diameter, set in a protective cone-shaped cavity in the skull called the orbit or socket.
• The thick white outer covering of the eyeball is called the sclera.
• The eyeball has a bulge, called the cornea, on its front surface.
• The space behind the cornea is filled with a clear liquid called the aqueous humor.
• Just behind the cornea is the iris. The iris is the colored area with a hole in the center called the pupil.
• Just behind the iris and pupil is the lens.
• The light-sensitive layer that lines the inside of the eyeball is called the retina. The retina is covered by rods and cones.
• The inside of the eyeball is filled with vitreous humor.
• The optic nerve is the bundle of nerve fibers in the back of the eye that leads to the brain. Six muscles surround the eyeball and help it to move.

Orbit — The orbit is surrounded by layers of soft, fatty tissue which protect the eye and help it to easily turn.
Cornea — This clear “window” helps to focus light rays going to the retina in the back of the eye. The cornea and the lens work together.
Iris — Tiny muscles in the iris allow it to dilate (widen) and constrict (narrow) the pupil size. This regulates the amount of light that enters the eye.
Lens — This is the clear flexible part of the eye that adjusts the focus of light rays that have passed through the cornea and helps us to see objects both near and far. The cornea and the lens work together to ensure the retina receives a focused image.
Aqueous humor — This water-like fluid maintains the eye pressure and allows focusing of the lens.
Retina — The retina captures the light images received by the lens and the cornea, converts them to electrical (nerve) impulses, and sends them to the optic nerve.
Rods — Rods are the light-sensitive cells that allow us to see black and white, and work best in dim light.
Cones — Cones are the light-sensitive cells that allow us to see color, and work best in bright light. Rods and cones cover the entire retinal surface.
Vitreous humor — This is a jelly-like substance that cushions the eye and gives it its shape.
Optic nerve — The optic nerve consists of a bundle of about one million nerve fibers. It delivers electrical impulses coming from the retina to the brain for processing.
Brain — In the brain, the signals are converted back from nerve impulses to images.

Eyelids protect the eyes from light and injury and provide moisture. They spread tears evenly over the cornea to keep it smooth.

Eyebrows and eyelashes protect the eyes from particles like dust.

Eyebrows also play an important role in expressing personality — they help to shape our expressions and how we show emotion.

Tears are formed by tiny glands that surround the eye. Tears are important because they keep the eye moist, maintain a smooth surface for light to pass through the eye, nourish the front of the eye, and provide protection from injury and infection.

The eye changes light rays into electrical signals that are sent to a special part of the brain called the visual cortex. The brain interprets these electrical signals as visual images. The eye is able to see in dim light or bright light, but it cannot see objects when light is absent. Watch this animation from the National Eye Institute to learn more.

When focusing images on the retina, your eye lens turns them upside down. Your brain then turns them right side up again. Your brain also needs to merge the two slightly different images from each eye into one. Your brain creates a 3-D picture. Learn more about 3-D vision.

Did you know that some people are color blind? The most common form is known as red/green color blindness, where people have trouble distinguishing either red or green. Males are more likely to be colorblind than females, and the condition is inherited from your parents. Take the color-blind test.

How do people “see” if they are blind? They use braille, a form of written language in which letters of the alphabet are represented by patterns of raised dots that are felt with the fingertips. Learn about the 6 dots of Braille.

Learn more at the Science Trek web pages for Light and Color and the Five Senses.

Do dogs see in color? Do cats see what we see? Do some animals have more than two eyes? How do animals see in the dark? Which animal has the largest eyeball on earth? Discover amazing facts about animal eyes.

Some animals have eyes facing forward. This gives them the ability to see their prey and keep them in sight during a chase. Some have eyes on either side of the head so they can keep a wide view of all their surroundings.

Joke — Did you hear about the cross-eyed teachers? They couldn't control their pupils!

Did you know that exposure to UV radiation can harm your eyes as well as your skin? Learn about the science and technology of sunglasses that can protect your eyes.

Here's a tip for tired eyes: Blink several times to wash your eyes with tears.

Learn about the differences among the people who take care of your eyes. They are ophthalmologists, opticians, and optometrists.