ABSTRACT
In the most common case, when the medium surrounding the lens is less dense than the lens material, a lens thicker at the center than at the edge is convergent, and a lens thinner at the center is negative. If the lens has a plane surface, the optical axis is the line that passes through the center of curvature of the spherical surface and is perpendicular to the plane surface. Obviously, this axis passes through the thickest or thinnest part of the lens. It is not possible with a diverging lens to form virtual images with real objects. When moving a virtual object, near the focus, there is a singularity. On one side of the focus, the image is real, and on the other side, it is virtual. Additional details on the subject of first-order paraxial optics may be found in Hopkins and Hanau.
