bifocal eyeglasses have two distinct optical powers so that they can help correct both nearsightedness and farsightedness. They are very useful to people who have both myopia and presbyopia or astigmatism. But bifocals may cause headaches or even dizziness in some new users. It will take some time for them to adjust to it.
Dude, bifocal eyeglasses can correct both nearsightedness and farsightedness. The lenses of bifocal eyeglasses are bigger than regular eyeglasses because they have two distinct optical powers. Each optical power help correct a kind of vison problem.