FOCUS!
When ligh rays enter our eyes, they must come to a point on the retina for a clear image to form. But the light rays that objects reflect or give off don't naturally move toward one another; The either spread out or travel almost parallel. The cornea and the lens (the focusing parts of our eye) bend the rays toward one another. The cornea provides most of the refracting (bending) power of the eye. After the light rays pass through the cornea, they travel through the aqueous humor and the pupil to the lens. The lens bends the rays even closer together before they go through the vitreous humor and strike the retina. When you aim your eyes to an object, its light rays come together at the fovea centralis (a tiny pit in the center of the macula) It is the area of sharpest vision. Light rays from objects to our sides strike other areas of our retina.
As our eyes shifts focus between nearby objects and distant ones, the refracting power of the lens change constantly. Light rays from distant objects travel nearly parallel, and light rays from nearby objects spread out. Therefore, our lens must provide greater bending power for the light rays from nearby objects to come together. Additional power is produced by a process called accomodation. In this process, one of the muscles of the ciliary body contracts, thereby relaxing the fibers that connect the ciliary body to the lens. The lens become rouder and thicker, and more powerful as a result. When our eyes look at distant objects, the muscle of the ciliary body relaxes. This action tightens the fibers that are connected to the lens, and the lens becomes flatter. For this reason, the eye can't form a sharp image of a nearby object and a distant one at the same time.
Our Depth Perception
Depth Perception is the ability to judge distances and to tell the thickness of objects. The lens ystem of the eye, like the lens of that camera dad gave you for your birthday, reverses images. So the images that form on our retina are very similiar to those produced on the film of a camera. The images are upside down and reversed left to right. They're also flat, just like in a photograph. However, our brain interprets the images as they really are. The ability of our brain tp interpret retinal images right-side up, unreversed, and in depth comes from experience that begins at your birth!
The optic chiasm is the point at the base of the brain where the optic nerves from our two eyes meet. This is where half the nerve fibers from each eye cross over and join the fibers from the other eye. Each side of the brain recerves visual messages from both eyes. The nerve fibers from the right half of each eye enter your left side of the brain. These fibers carry visual messages from objects that are to a person's right. Thus, if one side of the brain becomes damaged, the oppsite side of a person's feild of vision may be reduced. Such damage may occur as a result of a stroke, tumor, or injury.
Our eyes are about 2.5 inches apart from center to center. Because of this, each eye sees things from a slightly different angle and sends slightly different messages to the brain. The difference can be demonstrated by focusing on a nerby object first with one eye closed and then with the other eye closed. See? It's a little different isn't it? Our brain puts images together and thus provides depth perception, or stereoscopic vision, orrrrr three-dimensional vision (3-D!!). The image formed by our brain has thickness and shape, and the brain can judge the distance of the object.
"Normal" depth perception requires that the eyes work tgether in a process called binocular vision or fusion. In this process, the eye muscles move the eyes so that the light rays from an object fall at a corresponding point on each retina. When viewing objects close up, our eyes turn slightly inward (Haha! Haha! Cross-eyed! Cross-eyed!). When viewing distant objects, the eyes are almost parallel (I got nothing for that one). If images don't fall at a corresponding point on each retina, they will be blurred or be seen as double, or the brain will ignore one of them.
For most people, visual messages are stronger in one eye and on one side of the brain than the other. Most people are "right-eyed" or "left-eyed" - that means that they only have just a right eye, or just a left eye.....Just kidding. It means they favor one eye or the other when aiming a camera or rifle. Just like you favor one hand over the other in writing and throwing.
Just Give Me the Light! Or the Dark.
Adaption to light and dark is partly controlled by the pupil. In strong light, your pupil may become as small as a pinhead! Haha!! This prevents your eye from being damaged or dazzled by too much light. In the dark, it can get almost as large as the entire iris, thus letting in as much light as possible. However, the most important part to adaptation to light and dark occurs in the retina.
Light rays are absorbed by pigments in the retina's rods and cones. These pigments consists of proteins and vitamin A. Vitamin A helps give the pigments their color. The color enables the pigments to absorb light. Light changes the chemical structure of the vitamin A and bleaches out the color in the pigments. This process generates an electrical signal that the optic nerve transmits to the brain. After the pigments have been bleached, the vitamin A moves into a part of the retina known as the retinal pigmented epithelium (RPE). The vitamin regains its original chemical structure in the RPE and then returns to the rods and cones. There, it joins with protein molecules and forms new pigments.
The renewal of rhodopsin (the pigment that enables the eye to see in dim light) occurs largely in the dark. Immediately after being exposed to bright light, the eyes can't see well in dim light because of the bleached rhodospin. It takes about 10-30 minutes for the rhodospin to be renewed, depending on how much it was bleached. During this time, the eyes become accustomed to the dark.
The cone pigment, which provide sharp vision in bright light, take less time than rhodospin to be renewed. The eyes become accustomed to bright light much quicker than they do to darkness. The adaptation from darkness to light depends largely on changes in the retina's nerve cells.