PHYS102: Introduction to Electromagnetism

The image that needs to form on our retina to have clear vision is called a real image because it has rays that originally diverged from one point on the object coming together at one single point in the image. But depending on the object distance and the shape of the lens, one can also observe virtual images. A virtual image is formed when light rays emitted from a single point on the object come out the other side of a lens in a diverging fan which would meet at a single point if traced back to the input side of the lens along straight lines.

Watch this view which explains how virtual images occur in concave lenses.