A concave lens is a type of lens that is thicker at the edges and thinner at the center. A 20cm concave lens would have a diameter of 50 mm and a focal length of 20cm.

The lens would have a concave surface on both sides, meaning that it would diverge incoming light rays. When light rays pass through a concave lens, they are refracted or bent away from the center of the lens. This causes the image to appear smaller and virtual.

The power of a concave lens is negative, indicating that it causes light rays to diverge. The strength of the lens is measured in diopters, with a higher diopter indicating a stronger lens.

A 20cm concave lens is a type of lens that causes incoming light rays to diverge and is commonly used in vision correction.