How do glasses correct your vision? I know it has something to do with the retina but what exactly does it do?
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A lens works by refraction: it bends light rays as they pass through it so they change direction. That means the rays seem to come from a point that’s closer or further away from where they actually originate—and that’s what makes objects seen through a lens seem either bigger or smaller than they really are.
Our eyes rely on refraction, an effect where light changes direction when it enters a material that slows it down. By shaping the material right, you can form a lens, which focuses the light down to a point.
Depending on what direction the light comes from, this focal point moves, and so you can produce an image. This is how our eyes work.
The trouble is, the lens has to be calibrated for the right distance. If the light’s source (where it is scattered from) is too near or far, that image will be blurry. To account for this, the lens in your eye can change shape, but there is a limit to how much it can change shape. Some people’s lenses cannot change shape enough to have distant objects in focus.
By adding another (appropriately-calibrated) lens, we increase the focus distance, and allow those people to see with clarity once again.
You may recall from sr. elementary school or early high school, some diagram that shows light rays coming in one side of a lens. And how due to _refraction_ (the boundary between two different materials causing light to bend its direction), a properly shaped lens can focus light to a point? [A diagram like this.](https://cdn4.explainthatstuff.com/convexlens.gif)
Well, your eyeball works pretty much the same way. Instead of film or a chip sensor like in a camera, the thing that registers or records the image is your retina at the back of your eye. [So the lens and shape of your eyeball focuses light coming in so it hits your retina just right.](https://static.sciencelearn.org.nz/images/images/000/000/053/original/Eye-focus-final-3000X2000.jpg?1522292099)
Now, as your body grows and ages, sometimes the shape of your eyeball shape goes wonky, and the image isn’t focused on your retina quite as well as it used to. Or maybe you find yourself squinting – squinting smushes your eyeball so it’s closer to the right shape. So an optometrist, using various optical tools (but really they’re all just lenses) like the Photoroptor (“which is better, A… or B?”) can determine what type and shape of lens you need that will focus the light perfectly on your retinas.
When light is emitted (or reflected), it spreads out the further away it gets from its origin. Think a shotgun, scattering its shot in an expanding cone away from the barrel.
In order to focus the light into a single coherent image, your eye has a lens which redirects incoming light. When working properly, light from a single point on an object will be redirected to land on a single point on your retina, regardless of the angle it entered your eye. (In the shotgun example, this would be like all the shot being redirected in midair to to come back together).
However, if your eye isn’t working just right (irregularity of shape or not being able to flex the muscle to adjust the lens properly, etc.), the light won’t come together in exactly the right way. Light from a single point on an object will be spread out over a larger area on your retina, causing the image on your retina to be *out of focus* or blurry. Corrective lenses like glasses or contacts compensate for the for your eye’s shortcomings to bring the image into focus again.
Imagine using a magnifying glass to burn some dried leaves. To concentrate the sunlight, you would move the magnifying glass back and forth until all that light hits a single point.
In a similar way our eyes need to bend the light coming into it so that the light rays can concentrate at a point at the back of our eyes (the retina) where the light-sensing cells are.
However the amount we need to bend light changes depending on how far the object we are looking at is. Light from closer objects need to be bent more than light from further objects. Our eye has a special lens (like the magnifying glass) to bend the light. To adjust how much the lens bends light, we have special muscles to pull or contract the lens which increases or decreases how much we bend the light (like using our arms to move the magnifying glass back and forth). However these muscles can weaken over time which prevents the eye properly bending the light to our retina.
Glasses compensate for this muscle weakness by bending the light a bit before it gets to your eyes so the muscles don’t have to do as much bending to get the object in focus.
This is also why people need different prescription lens – since some people will have weaker muscles than others and therefore they will need glasses that can do more (or less) of the light bending. And if a person has trouble seeing close things (myopia), they need glasses to help concentrate light (basically a magnifying glass) while a person who has trouble seeing far away things (hyperopia) will need glasses to help spread out light (the opposite of a magnifying glass). We have even designed glasses that can concentrate and spread light (bifocals) for people who have trouble with both.
When light bounces off an object, it scatters in different directions. Some of these rays will be able to go through your pupil and hit your retina. But, if that’s all that happened, your vision would be blurry. Instead, you have a lens which can bend these rays together so that all the rays coming from a particular point in space hit the same point on your retina. This means there is a picture projected onto the back of your eye.
But, different distances will require different amounts of bending. Imagine two people standing a meter apart. If you’re a covid-safe 2m away from them and try to point to one with one arm and one with the other, there’s an angle between your arms. This is like the angle between light going through the top of your pupil and the bottom of your pupil. Now say you’re at the opposite end of a football field from them (whichever version of football) and you point your arms at them. Your arms are basically pointing in the same direction. This is the difference between the light from near and far objects.
The lens in the eye adjusts how much bending it does by changing shape. But, because of reasons, it might not be able to do it enough, meaning nearby objects appear blurry. In this case, glasses have a convex (bulging) lens that does some of the bending. Or, your eyes might be stuck bending light too much, meaning far objects are blurry. Then, glasses have concave lenses that actually bend the light outwards, meaning the extra inward bend your eyes do is now the right amount.
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