A unit of measure of the refractive power if a lens. A one-diopter lens will focus parallel light rays one metre from the lens and a two-diopter lens will focus one-half of a metre from the lens. A plus 1.0 diopter lens is convex and will converge the light rays so they focus as a visible image 1 metre past the lens. A minus 1.0 diopter lens is concave and will diverge or spread light. The minus lens will not actually focus as a visible image on an optics table. Its image is known as a virtual image and if the diverging rays were followed to their point of origin, they would focus one metre in front of the minus lens.