Short and long sightedness are caused by the lens of the eye
focusing the light either in front of or behind the retina. Glasses
alter the path of light going into the lens to adjust for this.
Bifocal and trifocal lenses solved problem for people who have
multiple vision troubles. They could have one pair of glasses for
reading and distance. They did not have to carry around several
sets of glasses. They even have bifocal and trifocal sunglasses
now.