How can a piece of glass in front of your eye fix blurry vision?

A corrective lens fixes blurry vision by bending light before it enters your eye, so that it lands exactly on the retina instead of just in front of or behind it. The eye's own optics are slightly off, and the lens makes up the difference. A person who is short-sighted needs a lens that spreads light a little. A person who is long-sighted needs one that gathers it.

In terms of shape, glasses for farsightedness are convergent, which means they bring light rays together. For short-sightedness the lens is the opposite, a divergent shape that spreads them out. To correct presbyopia, the age-related loss of close focus, a lens or part of it must be more convergent than the person's distance lens. Strengths are measured in quarter-diopter steps, because most people can't tell smaller differences.

The history is full of surprises. The earliest ancestors of glasses were 13th-century Italian reading stones, magnifying spheres without arms that you held directly over the page. The temple arms that hook over the ears were not invented until the 1700s. Concave lenses for short sight appeared in the mid-15th century, but nobody could explain why any of this worked until Johannes Kepler did it in 1604.

Benjamin Franklin, who had both myopia and presbyopia, invented bifocals, which let a person see clearly at distance and near without taking the glasses off.

Contact lenses took longer. The first successful one was made in 1888, and the large, heavy blown-glass lens could be worn for only a couple of hours. Today over 150 million people use them. They give a wider corrected field of view than glasses, but carry a higher risk of infection.