I have to do a project about it in school.
How do they focus the light onto the retina?
They change the angle of the light, so that it hits either at a point just behind, or just in front of the retina (depending on whether they are being used to treat long or short sightedness).
They use something called Refraction to bend the light so it focuses at a different place inside the eye. The material the lenses are made from have a certain amount of Refraction which changes the speed of the light through the material. The distance from where the light enters the lens and where it exits the lens to go into the eye determines how much the light is bent. So if you are only a little myopic they give you thin lenses that only bend the light back a short distance to fall in focus on your retina. If you are really myopic then they give you thicker lenses that bend the light farther back into the eye to focus on the retina. So the more myopic you are the longer your eyeball is and the more the lenses have to bend the light farther back into the eye. With very myopic eyes they use material with a higher refraction so the lenses can be thinner while still bending the light far back to hit the retina.