What Are Bifocal Eyeglasses and Their Types?

Bifocal eyeglasses are not a new comer. They have existed in the market for a long time since Benjamin Franklin first invented them in the 1760s. however, a lot of eye wearers still have no idea of what are bifocal eyeglasses. To be honest, I only knew them a few days ago.
You can tell some features of bifocal eyeglasses from their name. Bifocal is a compound word, “bi” meaning two, and focal referring to the focal length. Bifocal glasses have two focal lengths. One focal length allows the wearer to focus at distant objects. The other focal length allows the wearer to focus on closer objects, such as a newspaper or computer screen.
Bifocal glasses are a way for your eye doctor to provide you with a tool to help you see at a distance and up close.
When we’re younger a normal eye has the ability to change it’s focal length. When everything is “young” and working properly, we can alter the shape of the clear lense inside our eye thereby changing the focal length whenever we want to look at something near or far. As we get older, we loose that ability. Somewhere around 45, most of us start to have trouble focusing on closer objects. That’s because the lense INSIDE our eye can no longer change shape on demand the way it did when we were younger.
After knowing what are bifocal eyeglasses, it is time to know how many types do they have. There are different types of bifocals. They are:
(a) Solid bifocal: made from a single sheet of glass. A superb lens; no colour distortion; however, it is a bit costly. The very popular straight line bifocal falls in this group. Well worth the expense.
(b) Split bifocals, called Franklins from their 18th century inventor. Actually 2 lenses cut separately and put in one frame; extremely poor visual result; their only advantage and really the reason for their survival being that they are extremely cheap.
(c) Fused bifocals: a good commercial lens. A small circle is ground on the distance lens and another piece of glass of a higher refractive index (and therefore higher converging power) is put in the gap. It is fused in an electric furnace.
(d) Cemented bifocals: a small wafer of glass is cemented or glued at the bottom. A technique used around 30 years ago with disadvantages such as poor vision at the edges and that the joint gets yellow and bubbles appear. At present, used purely as temporary device or for matching unlikely numbers; purely of historical interest; again cheap.
What are bifocal eyeglasses? How many types do they have? They offer you a general knowledge about bifocal eyeglasses. You can still see from these some advantages of bifocal eyeglasses. however, if you want to know more about their advantages, you should search online for more information.