How do glasses work and what is the physics behind them? — SDM, Missouri
Like a camera, your eye collects light from the scene you’re viewing and tries to form a real image of that scene on your retina. The eye’s front surface (its cornea) and its internal lens act together to bend all the light rays from some distant feature toward one another so that they illuminate one spot on your retina. Since each feature in the scene you’re viewing forms its own spot, your eye’s cornea and lens are forming a real image of the scene in front of you. If that image forms as intended, you see a sharp, clear rendition of the objects in front of you. But if your eye isn’t quite up to the task, the image may form either before or after your retina so that you see a blurred version of the scene.
The optical elements in your eye that are responsible for this image formation are the cornea and the lens. The cornea does most of the work of converging the light so that it focuses, while the lens provides the fine adjustment that allows that focus to occur on your retina.
If you’re farsighted, the two optical elements aren’t strong enough to form an image of nearby objects on your retina so you have trouble getting a clear view while reading. Your eye needs help, so you wear converging eyeglasses. Those eyeglasses boost the converging power of your eye itself and allow your eye to form sharp images of nearby objects on your retina.
If you’re nearsighted, the two optical elements are too strong and need to be weakened in order to form sharp images of distant objects on your retina. That’s why you wear diverging eyeglasses.
People are surprised when I tell them that they’re nearsighted or farsighted. They wonder how I know. My trick is simple: I look through their eyeglasses at distant objects. If those objects appear enlarged, the eyeglasses are converging (like magnifying glasses) and the wearer must be farsighted. If those objects appear shrunken, the eyeglasses are diverging (like the security peepholes in doors) and the wearer is nearsighted. Try it, you’ll find that it’s easy to figure out how other people see by looking through their glasses as they wear them.