The term used to explain being short-sighted is myopia.
When looking at objects close to them a myopic individual has perfect vision, but distant objects can appear blurred.
This is because, in front of the retina, light is concentrated, and being so far ahead in the eye, objects appear distorted in the distance.
Using a concave (curved inward) lens that is positioned in front of a myopic eye, transferring the vision back to the retina and making it clearer, shortsightedness is corrected.