Bifocals are two prescriptions combined into a single lens.
Bifocals were originated by Benjamin Franklin in the 18th century when he cut the halves of two spectacle lenses and fitted them into one frame.
Bifocals are needed because the distance glasses do not suffice to focus sufficiently for near. As age increases, (in India 38-40 years is the presbyopic age) reading glasses are required to read at a comfortable distance. Rather than taking out the distance glasses and putting on near glasses each time, a person who wishes to work at the near point could use the lower segment comfortably.
There are various types of bifocals available, from the round-top bifocal, flat-top bifocal to the executive bifocal.
The lower half of a bifocal lens contains the near segment for reading and other close-up tasks. The rest of the lens is usually a distance correction, but sometimes has no correction at all in it, if you have good distance vision.
One popular fused bifocal is the round seg bifocal. Round-top bifocals are further described by the width of the near segment, measured in millimeters (mm). The most popular RT bifocal is the RT-28, a round-top bifocal with a near seg that is 28 mm wide.
An advantage of round seg bifocals is that the line separating the distance and near zones of the lens tends to be less noticeable than the upper line of a D-seg bifocal.