How glasses work?

One of the most common sights almost anywhere in the world is -- eyeglasses! Since we depend so much on the lenses inside those frames to improve our view of the world, you might wonder just what goes into creating them.

On the back of your eye is a complex layer of cells known as the retina. The retina reacts to light and conveys that information to the brain. The brain, in turn, translates all that activity into an image. Because the eye is a sphere, the surface of the retina is curved.

When you look at something, three things must happen:
- The image must be reduced in size to fit onto the retina.
- The scattered light must come together - that is, it must focus -- at the surface of the retina.
- The image must be curved to match the curve of the retina.

Out of Focus

Most vision problems occur when the eye cannot focus the image onto the retina. Here are a few of the most common problems:

• Myopia (nearsightedness) occurs when a distant object looks blurred because the image comes into focus before it reaches the retina. Myopia can be corrected with a minus lens, which moves the focus farther back.
• Hyperopia (farsightedness) occurs when a close object looks blurred because the image doesn't come into focus before it gets to the retina. Hyperopia, which can also occur as we age, can be corrected with a plus lens. Bifocal lenses, which have a small plus segment, can help a farsighted person read or do close work, such as sewing.
• Astigmatism is caused by a distortion that results in a second focal point. It can be corrected with a cylinder curve.

In addition, lenses can be made to correct for double vision when the eyes do not work together ("crossed eyes"). The lenses do this by moving the image to match the wayward eye.

When light pass through the lens, it is always bent toward the thickest part of the lens. To make a minus lens (above), the thickest part, the base is on the outer edges and the thinnest part, the apex, is in the middle. This spreads the light away from the center of the lens and moves the focal point forward. The stronger the lens, the farther the focal point is from the lens.

To make a plus lens (below), the thickest part of the lens is in the middle and the thinnest part on the outer edges. The light is bent toward the center and the focal point moves back. The stronger the lens, the closer the focal point is to the lens.

Placing the correct type and power of lens in front of the eye will adjust the focal point to compensate for the eye's inability to focus the image on the retina.

And by putting those lens in the frame, the glasses are created.