Virtual object

For an optical element, a virtual object is a point toward which incident rays already converge, so they would meet beyond the element if it were removed. It is distinct from a virtual image, which is an apparent origin of outgoing rays. In the real-is-positive thin-lens convention, a virtual object beyond the lens has $u<0$, and $1/u+1/v=1/f$. Thus $u\to -\infty$ gives a real image in the rear focal plane of a converging lens.