A concave lens has a thin middle and makes objects appear smaller. Its surfaces curve inward, so it is thinner at the center than at the edges. When parallel light rays pass through it, the lens bends them outward, or diverges them.
Because the diverging rays appear to come from a point behind the lens, a concave lens forms a virtual, upright, and reduced image when used by itself. This is why objects viewed through a simple concave lens appear smaller than they do with the unaided eye. The amount of reduction depends on the lens’s curvature and focal length.
Concave lenses are also called diverging lenses. They are used in glasses that correct nearsightedness, or myopia, because they spread incoming light slightly before it enters the eye, helping the eye focus the image on the retina. A common mix-up is with convex lenses: convex lenses are thick in the middle, converge light, and can magnify nearby objects.