Virtual object
= 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.