Top: The formation of a virtual image using a diverging lens. Bottom: The formation of a virtual image using a convex mirror. In both diagrams, f is the focal point, O is the object and I is the image, shown in grey. Solid blue lines indicate light rays. It can be seen that the light rays appear to emanate from the virtual image but do not actually exist at the position of the virtual image. Thus an image cannot be seen by placing a screen at the position of the virtual image.

In optics, a virtual image is an image formed when the outgoing rays from a point on an object always diverge. The image appears to be located at the point of apparent divergence. Because the rays never really converge, a virtual image cannot be projected onto a screen. In diagrams of optical systems, virtual rays are conventionally represented by dotted lines. Virtual images are located by tracing the real rays that emerge from an optical device (lens, mirror, or some combination) backward to a perceived point of origin.

In contrast, a real image is one that is formed when the outgoing rays from a point converge at real location. Real images can be projected onto diffuse reflecting screen, but a screen is not necessary for the image to form.[1]

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 exists in a space that is not real in a sense. 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.

Whenever we look through a diverging lens (one that is thicker at the edges than the middle) or into a convex mirror, we see 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) and concave mirror are also capable of producing virtual images 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 would produce a real image. Such an image may be magnified or reduced depending on the position of the object.