In optics, the image of an object is defined as the collection of focus points of light rays coming from the object. A real image is the collection of focus points made by converging rays, while a virtual image is the collection of focus points made by backward extensions of diverging rays. In other words, a virtual image is found by tracing real rays that emerge from an optical device (lens, mirror, or some combination) backward to perceived or apparent origins of ray divergences.
There is a concept virtual object that is similarly defined; an object is virtual when forward extensions of rays converge toward it.[1]
For a (refracting) lens, the real image of an object is formed on the opposite side of the lens while the virtual image is formed on the same side as the object. For a (reflecting) mirror, the real image is on the same side as the object while the virtual image is on the opposite side of, or "behind", the mirror. In diagrams of optical systems, virtual rays (forming virtual images) are conventionally represented by dotted lines, to contrast with the solid lines of real rays.
Because the rays never really converge, a virtual image cannot be projected onto a screen by putting it at the location of the virtual image. In contrast, a real image can be projected on the screen as it is formed by rays that converge on a real location. A real image can be projected onto a diffusely reflecting screen so people can see the image (the image on the screen plays as an object to be imaged by human eyes).[2]