Concave lenses, which are thinner at the center than edges, work by allowing parallel rays of light to pass through, refract and diverge, making the image appear to come from one point. This point is known as the principal focus. This effect also makes the image appear smaller.
Concave lenses makes images appear smaller, but they also make them appear farther away. Side mirrors on cars are slightly curved liked a concave lens, which is why they must always state "Objects in mirror are closer than they appear."
Shortsightedness, also known as myopia, is corrected using concave lenses. A person who suffers from myopia is unable to focus on distant objects with their eyes because the light is being refracted more than it should, says the Physics Classroom. This is typically caused by a bulging cornea or elongated eyeball. The concave lens corrects this by allowing the light to diverge before it hits the eye.
The Galilean telescope uses a concave lens as the eyepiece in order to help magnify objects in outer space, and this was an early use of these lenses.
The convex lens, which is thicker in the middle than its edges and makes images appear larger, has the opposite function of a concave lens.