It is said that your eyes are your windows to the world. However the actual act of “seeing” is much more complex than that common phrase would make it seem.
When you see an object what you are actually seeing are light rays reflecting off that object and into your eye. Your eye's complex components intercept, focus and process the light into nerve impulses, which are sent to the brain. This is how we "see."
Iris and Pupil
The iris is the part of your eye that gives it color (blue, green, brown, etc.) It functions like a shutter in a camera, allowing more or less light into the eye.
The pupil is the round opening in the middle of the iris.
Cornea and Lens
The cornea is the crystal clear dome that covers the front of your eye. When light passes through the pupil, the cornea bends (or refracts) the light rays.
The crystalline lens finishes the focusing of light. It helps to "fine tune" your vision.
Unlike the cornea, the lens can be made to change its shape (and therefore its refractive power) rapidly and voluntarily. By changing its shape, the lens allows your eyes to change focus and see objects that are near.
The retina is a thin layer of nerve tissue at the back of your eye that senses the light. Specialized cells called rods and cones convert light energy into nerve signals that travel through the optic nerve to the brain. Your brain then produces an image and the act of “seeing” is complete.
The shape (curve) of the cornea, the power of the crystalline lens, and the length of your eye together determine how rays of light are focused on the retina.
When these 3 factors are sized and working appropriately, you can see clearly without the need for vision correction (glasses or contacts).
However, if some of these factors don’t match up, the result is vision impairment.
Nearsightedness (trouble seeing things that are far away) occurs when the cornea is too steeply curved or the eye is too long.
Farsightedness (trouble seeing things up close) occurs if the cornea is too flat or the eye is too short.
As you age, your crystalline lenses become stiff and lose the ability to focus on near objects (called presbyopia). This happens to everyone, usually around the age of 50, and results in the need for reading glasses, bifocals, contacts or a combination of these.