A virtual image (as opposed to a real image) is produced by an optical system (a combination of lenses and/or mirrors) when light rays from a source do not cross to form an image. Instead they can be 'traced back' to a point behind the lens or mirror. Virtual images can be seen directly without using a screen for projection.

The most common virtual images are those produced by a flat or plane mirror. As shown in the diagram below, the virtual image appears the same distance behind the plane mirror as the object is in front of the mirror.

The virtual image produced by a plane mirror.

The virtual image produced by a convex lens when the source is closer to the lens than the focal length, f. Note that the virtual image is magnified and upright compared to the object.