Bifocal glasses refer to those having one section that corrects for distant vision and another section that corrects for near vision and Benjamin Franklin is believed to be the one who invented bifocals. The reason why it is called a bifocal derives from its special designation of the two lenses of different convexes positioned in respectively the lower and upper half of the frame. At the very beginning the two parts were just combined directly, resulting in a sharp switch for the wearing when s/he read something from the above or from the top. Later, to solve this problem, at the end of the 19th century, Louis de Wecker developed a method for fusing the sections of the lenses together.
Bifocals were originally invented for the elders to read, and then people think that only aging people require bifocals their development of presbyopia all the time, but this is a misconception. The fact is that bifocals are also suitable for those who develop a combination of nearsightedness and farsightedness. For them, the lenses posited in the lower half of the frame are for nearsightedness correction while the upper are for farsightedness correction. Therefore, when you find young friends around you wear bifocals, do not feel surprised.
Based on the same principle, bifocals can help correct any combination of two different eyesight conditions, for example, bifocals can help correct astigmatism that is present in combination with other refractive defects. However, every coin has two sides, bifocals lead to discomfort for the sharp vision switch from the upper to the lower or in reverse as mentioned above. Some wearers may experience from headaches and even dizziness. For the computer users, they would probably experience muscle fatigue due to the unusual angle and constant movement of the head.
Fortunately, with the progress of relative researches on it, new models come into being, the trifocal glasses and progressive glasses. As their names self-explanatory, the former consist of three corrective lenses on each side of a pair of glasses, for a total of six lenses for a pair while the latter are characterized by a gradient of increasing lens power. Both of them work similarly to bifocals do, but they aim at reducing the sharpness between different corrective lenses so as to make wearers suffer from fewer headaches from strained eyes.
Those progresses are not enough, researchers are trying to make new materials to make better bifocals, and to discovery new technologies that may provide a method which can selectively adjust the optical power of a lens. At last, I would like to remind you that if you find yourself a need to wear bifocal glasses, you can firstly consult with an optician, who is best to be licensed, for there are different types of bifocals. With her/his help, you can decide which type of is the one for you.