What causes people to have to wear glasses?

From "Neuroscience for Kids"

Do You Wear Glasses? Here's Why!

Have you ever seen this "eye chart" before? It is smaller than the real version of the chart that is used to test how well you can see. It was first developed in 1863. If you have 20/20 vision, it means that at a distance of 20 feet, you can read a certain line (labeled 20) on the chart and that your vision is normal. If you can only see the top line clearly (the one labeled 200), then you have 20/200 vision. This means that you must be 20 feet from the chart to see what most people can see at 200 feet. By the way, if someone's vision is 20/200 or worse, then they are legally blind.

People wear glasses for several reasons. People who have "fuzzy" vision when they look at distant objects are called nearsighted (myopia). In these people, the image is focused in front of the retina. The lens of glasses needed to correct vision in nearsighted people is concave. People who need glasses for reading and who have "fuzzy" vision for objects close to them are called farsighted (hyperopia). In farsighted people, objects are focused behind the retina. The lens of glasses needed to correct vision in farsighted people is convex. Astigmatism is another reason that you might have to wear glasses.

So what causes myopia? There are many different causes. When the eye is too long it results in Axial Myopia. When an eye is too strong because the cornea or the lens is too strong then you have Refractive Myopia . Either way, the treatment is glasses, contacts or Refractive Surgery.

So what causes hyperopia? Like myopia, there are many different causes. When they eye is too short it results in Axial Hyperopia. When the eye is too weak to focus the image on the retina it results in Refractive Hyperopia.

Astigmatism can be caused by the cornea, the lens or both. Small amounts of astigmatism occur naturally, but larger amounts start to cause blurriness, smeared images or shadowed images.