The geometric point behind a diverging optical element where light rays would intersect if traced backward along their paths. Because the actual photons spread outward and never cross at this location, the focus cannot be captured on a physical surface. It is a mathematical reference point used to calculate the focal length of concave lenses and convex mirrors.
Heard on the job
The plano-concave lens has a virtual focus at minus fifty millimeters.
Related terms
Virtual
Describing an optical point or image formed by the apparent divergence of light rays rather than their actual intersection.
Virtual image
An optical representation formed by diverging light rays that cannot be projected onto a screen.
Focal point
The specific point in space where collimated light rays converge after passing through a lens.