Virtual image

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 real converging rays, while a virtual image is the collection of focus points made by backward extensions of real diverging rays. 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 real ray divergences. In other words, a virtual image point is from which the real rays appear (not actually) to be emitted (while a real image point is to which real rays converge).

There is a concept virtual object that is similarly defined; an object is virtual when forward extensions of real rays converge toward it. In other words, a virtual object point is to which the real rays appear (not actually) to converge (while a real object point is from which real rays emit). This is observed in ray tracing for a multi-lenses system or a diverging lens. For a diverging lens, forward extension of real converging rays toward it will meet the converging point, so the point is a virtual object.

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 are not actually present at the apparent location, and thus never really converge at any point, 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).

• A plane mirror forms a virtual image positioned behind the mirror. Although the rays of light seem to come from behind the mirror, light from the source only exists in front of the mirror. The image in a plane mirror is not magnified (that is, the image is the same size as the object) and appears to be as far behind the mirror as the object is in front of the mirror.
• A diverging lens (one that is thicker at the edges than the middle) or a concave mirror forms a virtual image. Such an image is reduced in size when compared to the original object. A converging lens (one that is thicker in the middle than at the edges) or a convex mirror is also capable of producing a virtual image if the object is within the focal length. Such an image will be magnified. In contrast, an object placed in front of a converging lens or concave mirror at a position beyond the focal length produces a real image. Such an image will be magnified if the position of the object is within twice the focal length, or else the image will be reduced if the object is further than this distance.