I have horrible eye sight, so that's why I need glasses to see better. And yet I never really understood how a mirror could make you see better. Until last year, when we learned in physics of optical glasses. Our eyes are concave, and the distance between our cornea to our retina is the focal length. So the light that hits our eyes allows us to see images. People who have twenty/twenty visions have eyes that aren't small or elongated and the focus is at the retina. This allows people to see images clearly. However people who have bad eye sight can either be far-sided or near-sided. The people who are far-sided, have their focus behind the retina because the eyes are short, therefore convex lenses can fix this. Convex lenses has a positive focal length which moves the image of a person far-sided forward. People who are near-sided, have their focus in front of the retina because the eye is elongated. Concave lenses can fix that because tit has negative focal length so it moves the image back to focus.