September 21, 2004

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The Eye's Iris and Pupil
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Brown Eye
Sight is the most precious of the five senses of sight, hearing, smell, taste, and touch. We rely on this sense more than any other. The human eye gives us the sense of sight. We use our eyes in almost everything we do. If you are curious about your eyes then this is the place for you!

Eyeball Anatomy

          Parts of the EyeTo understand how the eye works we'll first look at all the parts. This is called the anatomy of the eye. Click on the eye image to see a larger version.

  • 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 and help it to move. Wow!
    Learn more...
The orbit is surrounded by layers of soft, fatty tissue which protect the eye and help it to easily turn.
This clear "window" helps to focus light rays to the retina in the back of the eye. The cornea and the lens work together.
Tiny muscles in the iris allows it to dilate (widen) and constrict (narrow) the pupil size. This regulates the amount of light that enters the eye.
The clear flexible part of the eye that focuses light rays on the back of the eye and allows us to see both near and far. The cornea and the lens work together to focus images on the retina.
Aqueous humor
This water-like fluid maintains the eye pressure and allows focusing of the lens.
The retina detects images focused on the back of the eye by the lens and the cornea and sends them to the optic nerve. These light impulses are then sent to the brain for processing.
Rods are the light-sensitive cells that allow us to see black and white, and work best in dim light.
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

They translate this information to nerve impulses and send them to the brain.

The optic nerve is the structure which changes images to electrical signals and delivers them to the brain. The optic nerve consists of a bundle of about one million nerve fibers.

Brain In the brain, the signals are converted back from nerve impulses to images.

     Here is one more diagram to help you remember the parts of the eye.

 Outside the Eyeball- - eyelid, eyebrow, eyelash, tears

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

Eyebrow illustrationEyebrows and eyelashes protect the eyes from particles like dust.

Eyebrows are an important important part of our expressions and help show our emotions.

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

How eyes use light to see and send signals to the brain --

This animation from the BBC Science and Nature is the place to begin.

     The eye changes light rays into electrical signals that are sent to a special part of the brain. 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.

     By focussing images on your retina, your lens turns them upside down. Your brain turns them the right way up again.
      Your brain also needs to merge the two slightly different images from each eye into one. Your brain creates a 3D picture. More about 3D vision...

Tease your eyes and brain with these demonstrations.
 framing demonstration
 eyehop demonstration
Find out about and find your own Blind Spot Test

Did you know that some people are color blind? They can't tell the difference between two colors. Males are more likely to be colorblind than girls and the condition is inherited from your parents.
Take the color blind test.

How do people "see" if they are blind? Learn about the 6 dots of Braille  

Important dates in vision.

Animal VisionGecko Eye

     Some animals have eyes facing forward. This gives them a 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. Continue your exploration of animal eyes at the San Diego Natural History Museum and maybe these questions can be answered...

Do dogs see in color? Do cats see what we see? Do some animals hav
e more than two eyes? How do animals see in the dark?

See the world through the eyes of a Honeybee.  

Discover the differences among the people who take care of your eyes. What is an...
  Did you know that exposure to UV radiation can harm your eyes as well as your skin? Learn about different kinds of sunglasses that can protect your eyes.

Tip for tired eyes:
Blink several times to wash your eyes with tears.
More tips for keeping your eyes healthy.
Try these 3D Exercises for the eyes
Enjoy your work-out!

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