Structure

    The eye is a very complex organ that sends a huge amount of information to the brain. It has a very specific design to capture and analyze light. In its simplest description, the eye is a box, with a lens to focus the light that enters it, and cells to process the light.

    Almost the entire exterior of the eye is a"light-tight" box. Its outer walls are formed by a hard, white substance called sclera. The outside of the eye is light-tight so that light can only enter through a small opening. This produces clearer vision, because a smaller opening, or aperture, creates a sharper image.

	Prove it to yourself: Take an index card and puncture it using a pin. Be careful not to allow the hole to be too large! Look at something just far enough away that you cannot focus clearly on it. Hold your index card up to your eye and look through the tiny hole. While the aperture, the hole, blocks most of the light hitting your eye, it makes the far away object look clearer!

The eye is filled with two liquids. These provide nourishment to the other cells in the eye, just as blood vessels provide nourishment to most cells in the body. The difference between these liquids and blood is that they are nearly transparent, so they can nourish the cells of the eye without interfering with the light that enters. The two liquids in the eye are called the vitreous humorand aqueous humor.

	Prove it to yourself: Rapidly move your eyes (NOT YOUR HEAD!) back and forth. Suddenly stop moving your eyes, close them, and look at a bright light. You should see what appears to be little bubbles or chains floating across your closed eyes. These are dead cells floating in your vitreous and aqueous humors. The older you are the better this experiment will work. If you are young or have healthy eyes, try moving your eyes for a longer period of time.

 Focusing

    Light entering the eye is focused by two lenses: the cornea and the eye lens. The lenses hold their shape due to pressure from the vitreous humor and aqueous humor, as well as a muscle group called the ciliary muscles. Light is bent by the lenses to focus at the back of the eye. You can learn more about the bending of light in refraction.  As objects get further away from the eye the ciliary muscles relax, allowing the eye lens to become flatter and bend the light differently. Sometimes, due to age or genetics, the ciliary muscles will not bend the eye lens correctly, causing a blurred image. This condition is called either myopia (nearsightedness) or hyperopia (farsightedness).

		As an object moves closer to or further from the eye, the ciliary muscles adjust the shape of the eye lens so that the light is always focused to the same point on the back of the eye.

The Image

    The image created by the eye is real and inverted. Many people are surprised to learn that the images we always see are inverted. The reason we do not notice this is that we know no other reality. Studies have been performed where subjects have worn special goggles that distort their vision in certain ways. After along period of time, the brain accommodates for the goggles, and the subjects are able to do everyday tasks without difficulty!

	Don't Believe Us? Very carefully, place your finger on the side of your nose, next to your eye. Very lightly (be careful, extreme or prolonged pressure may cause eye damage) press on the inside of your eye. Notice a black spot appears on the outside of your vision! Because the image is inverted, what appears to be a sensation on the outside of your eye is actually a sensation on the inside! Still don't Believe us? Go into a dark room with a friend, and have him or her look at a small light-emitting object, like a candle. If the room is dark enough, you will be able to see up to three images in his or her pupil. The first image (upright and brighter than the other images) is a reflection off the cornea. The second image (upright and very dim) is a reflection off the eye lens. The third image (dim and inverted) is a reflection off the retina. This third image is the image that is sent to the brain!

Light Control

    One of the wonders of the human eye is that it can respondto a wider range of light than any artificial device ever created. In otherwords, it is possible to see not only in very low light levels (such as a darkroom) but also in very high light levels (such as a sunny day). In fact, thebrightest conditions under which an eye can operate are around 10¹³times as bright as the dimmest conditions.

    How does the eye do this? One way is by using the iris.The iris changes in size to allow different amounts of light to enter your eye.When there is more light, the iris shrinks. This blocks out much of the light,and as is demonstrated by the first activity, thisincreases the sharpness of your vision. If there is less light, the irisincreases in size, allowing more light to enter the eye to be processed.

	Prove it to yourself: There is a very simple way to see the iris at work. Stand in a dark room and look at a mirror. Suddenly turn on a light. You will see the iris, the black circle in your eye, quickly shrink as it adjusts to a greater level of light.

Go Back to The Eye

Move on to The Retina