Our eyes (like the rest of our bodies) are pretty clever. Light passes through the cornea and enters the eye through the pupil (a hole in our iris). It then passes through the lens which bends the light so that when it hits the retina – which is what does the detecting – the image is in focus.
With our new modern lifestyles our eyes (like the rest of our bodies) have got a bit lazy and in lots of people the lenses no longer focus the light properly. This leads to either short-sightedness (or myopia in which the light bends too much and things that are far away are blurry) or long-sightedness (hyperopia, where the light doesn’t bend enough and close-up things are blurry). Corrective lenses bend the light, either inwards or outwards, before it enters the eye so that by the time the light hits the retina it’s in focus, just the way it should be.
The four-eyes of the featured image is from Mo Riza and the anatomical view of an eye is from Holly Fischer, both used and edited with thanks under the Creative Commons License 2.0 and 3.0 respectively.