The Health Nut Blog

How Contact Lenses Work

A clear, relatively hard mound of tissue called the cornea sits in front of the pupil of the eye. Behind the pupil is the lens that focuses light onto the retina, which creates reactions that stimulate the optic nerve to send signals to the visual cortex in the brain. That, in a nutshell, is what makes sight possible. But along the way several things can go wrong.

With age (usually) or disease, or simply because of genetics, the shape of the lens and eyeball can and do become distorted. That distortion also affects the shape of the cornea. The change in shape causes the light rays to focus in front of the retina, behind it, or to be scattered in several directions.

In the first case, the result is nearsightedness or myopia. That is the ability to see things near, but far things appear out of focus. Reading a book is easy, reading road signs while driving can be difficult or impossible without contact lenses or glasses.

When the rays would focus at a point behind the retina the effect is to produce farsightedness, also called hyperopia. Farsightedness is the ability to see things sharply that are (relatively) far away, but things up close look blurry. Reading road signs is easy, reading text on your computer without contact lenses or glasses becomes impossible.

The third case is a little different from either of the other two. In this instance, the eyeball can be misshapen or out of round, making the cornea and lens an improper shape. That produces a condition called astigmatism, the effect of which is to make images appear blurry whether near or far. That double-whammy is one of the reasons that, until recently, it was difficult to produce contact lenses or use laser eye surgery to correct this.

But the solution is (today) straightforward. Insert another lens between the light rays and the human eye's lens. That works like a camera, which has multiple pieces of glass that can work together to focus the rays onto the film. In this case the 'film' is the retina. For centuries, this was done by using glasses, today, contact lenses offer an option enjoyed by millions.

The extra lenses (the contact lenses) 'float' on a thin layer of tear-like moisture over the cornea. Because of liquid surface tension and the shape of the eye - carefully matched by shaping the contact lens just right - the lenses can stay on and stay in place.

In short, in technical language, they change the refractive angle of the light. Refract is just a fancy word meaning 'to bend'. That's exactly what the lenses do. They bend the light a little bit. It's bent a little more by the cornea, lens and fluid in the eye and - when everything is adjusted just right - the light is focused sharply onto the retina. The net result is to provide the type of array the visual cortex can correctly process as 'car' or 'baby' or whatever you happen to be looking at.

# posted by NeilG @ 8:37 PM