Bifocal glasses work by correcting both near- and far-sightedness in an eye using two different sections of the lens, each having its own point of focus. For most bifocals, the nearsightedness is corrected in the top portion of the lens, while the farsightedness is corrected in a smaller portion of the lens toward the bottom, where one generally looks while reading. A double segment bifocal lens has a near or reading portion on the top and bottom of the lens.
Bifocals, trifocals and multi-focal lenses all have differing focal points in different portions of the lens. Most people look up when focusing on far away objects and look down when focusing on objects closer to them for reading. Bifocals typically correct this by adjusting the focal point of the wearer's vision for far away objects through the top portion of the lens and by adjusting the focal point for closer objects through a smaller portion of the lens near the bottom.
Trifocals adjust the wearer's vision into three focal areas: far, intermediate and near. Multi-focal or progressive lenses correct the wearer's vision using multiple focal points making the transition from near to far more smooth when going from near to far.