Bifocals are a type of eyeglasses that feature two different optical powers in one lens. The top part of the lens corrects distance vision, while the bottom part corrects near vision. The lenses are split into two parts, offering different prescriptions for various distances. The lower part of the lens can be one of several shapes, including a half-moon, round segment, or narrow rectangular area. Bifocals were first invented by Benjamin Franklin, who glued the top half of one lens to the bottom half of another. Modern bifocal lenses have evolved into more advanced designs, including flat-top bifocals and no-line or progressive lenses. Flat-top bifocals have a large area devoted to distance vision, with a small insert or segment for focusing on objects up close. No-line or progressive lenses have a smooth change in focus from the top of the lens (distance) to the bottom of the lens (reading distance). Bifocals are most commonly prescribed to people over the age of 40 who have difficulty seeing nearby objects and struggle with distance vision due to presbyopia.