Convex and concave lenses can both be used to correct defects of vision. The most commonly occurring defects of vision are ‘short sightedness’ and ‘long sightedness’.
People who are ‘short sighted’ cannot see things that are far away. They can only see things that are close to them. This is because the eye refracts the light and brings it to a focus before it reaches the retina. This is corrected by using a concave or a diverging lens.
People who are ‘long sighted’ can see distant objects clearly, but they cannot clearly see objects that are nearby. This is because the eye refracts the light rays and they are brought to a focus having passed the retina. This is corrected by using a convex or converging lens.