A convex mirror forms a diminished image, but a concave mirror, depending on the object's position, either forms an enlarged or diminished image. This is the Concave vs. convex lenses Concave lenses—those that are thicker around the edges than they are in the center— disperse light rays, which can have the effect of making things look smaller or farther away. Convex lenses—those that dome outward and are thicker in the center than they are around the edges—make light rays converge. Clearly if function is strictly concave upward (downward) then it is concave upward (downward). Now, let's see what is the geometric meaning of the concave functions. Without loss of generality assume that a
