Learning Object

Vertical Cross Section of the Eye

The eye is basically a hollow globe with a radius of a little less than an inch that is covered with a tough white coating, the sclera. Light rays entering the eye first go through the cornea, a transparent tissue, and the anterior chamber which is filled with a watery fluid called aqueous humor. Then they pass through the pupil, the circular opening in the iris that gets smaller when there is a lot of light, and larger when the light is dim. The pigments in the iris determine whether your eyes are blue, green, gray, or brown. Behind the iris lies the lens, which is about the size and shape of a small lime bean. The ciliary muscle surrounds the lens like a ring, and when it contracts, it squeezes the lens and makes it thicker. After passing through the lens, the light rays go through the fluid called vitreous humor that fills the hollow globe, and finally hit the retina at the inside back of the eye. The eye as a whole is quite a complex and amazing biological structure. But as a simplified model, you can imagine that cornea and lens act together like a single convex lens that produces an image of an object far outside of the eye. The screen where this image must appear, sharp and in focus, is the retina, or net. The retina is dotted with cells that can detect light, and these cells pass on the information they receive via the optic nerve to the brain. Remember how we showed in the last video and in the following animation that the image of a very distant object is located essentially at the focal point of the lens. This means that, in order to see distant objects in focus, the retina must be located at the focal point of the cornea/lens system of the eye. Of course the retina is attached to the back of the eyeball and therefore in a fixed position – the screen in this optics arrangement cannot be moved. But for a normal sighted eye, the eyeball has just the right size such that the focal point of the cornea/lens system is located on the retina. On the other hand, there are many people whose eyeballs do not have the exactly right length for their particular cornea/lens combination. However, if they combine their eyes with another lens, that is, if they wear eye glasses, they can correct their vision problems