Short-sightedness or myopia is a condition where distant objects appear blurred while close objects appear clear. This occurs when the light entering the eye is not properly focused on the retina but in front of it. To correct this, a concave lens is used. A concave lens is thinner at the center and thicker at the edges, and it diverges light rays. By placing a concave lens in front of the eye, it helps to focus the light rays properly on the retina, thereby correcting the myopia.