2. Refraction in the Eye
Vision, the capability we have of seeing something, is not as simple as you might expect. In fact, it is a result of the complex system of the eye, involving many processes. The main principle behind the functioning of the eye is that of refraction, which occurs four times between the instant a light ray reaches your eye and the moment an image is registered on the retina.
The amount of refraction that takes place when light passes from one medium to another is related to the difference in their refractive indexes. The greater the difference in them, the more refraction occurs. Therefore, it should not be too great a surprise that the greatest refraction in the eye occurs as the light passes from air to the cornea. Because the cornea is so dense compared to the air, light travels through it much more slowly than through the air. This means that their refractive indexes are quite different, and that the light that passes into the cornea it is bent a great deal. Upon entering the less dense aqueous humor, the light ray is bent away from the normal, but only slightly.
Because of its very rounded shape, the cornea acts very similar to a lens, and makes parallel light rays tend to converge. However, there is no point of convergence yet, because next in the path of the light rays is the lens. The lens is made of a fairly soft material, which means that its refractive index is nowhere near that of a glass lens, for instance. Also, because it is surrounded by liquid on both sides, the refraction carried out by it is not very great. Luckily, this is not the main function of the lens. Instead, it is the fine-tuning of the convergence of light rays. The ciliary muscles around the lens allow it to change its shape significantly, so that light rays from a point on an object are able to meet on the retina, no matter where the object is.
It is important to remember that the power of any lens also depends on the substance it is in. Part of the reason that the cornea has such great power is that it functions in air. When the eye is surrounded by another substance, as, for instance, water when you go swimming, the cornea has a greatly decreased power. This means light beams are not as easily made to converge, which is why everything appears blurry to us when seen underwater.
In the first diagram below, light rays coming from a nearby point, the ant, are made to converge on the retina by a fat lens:
On the second diagram, light rays from a faraway point are considered to be parallel, and are brought into focus on the retina by a thin eye lens:
In order for a point on an object to be totally in focus, the rays must meet exactly on the retina, a kind of "screen", from which a message is sent (via the optical nerve) to the brain. Because the eye lens is convex, the images it forms on the retina are all vertically inverted. If we registered directly what appeared on our retinas, everything would appear to be upside-down to us. The vertical interpretation is one of the many complicated processes carried out by our brain.