What is "virtual image"

Image \Im"age\ ([i^]m"[asl]j; 48), n. [F., fr. L. imago, imaginis, from the root of imitari to imitate. See Imitate, and cf. Imagine.]

1. An imitation, representation, or similitude of any person, thing, or act, sculptured, drawn, painted, or otherwise made perceptible to the sight; a visible presentation; a copy; a likeness; an effigy; a picture; a semblance.

   Even like a stony image, cold and numb.
   --Shak.

   Whose is this image and superscription?
   --Matt. xxii. 20.

   This play is the image of a murder done in Vienn

   1. 
      --Shak.

      And God created man in his own image.
      --Gen. i. 27.

2. Hence: The likeness of anything to which worship is paid; an idol.
   --Chaucer.

   Thou shalt not make unto thee any graven image, . . . thou shalt not bow down thyself to them.
   --Ex. xx. 4, 5.

3. Show; appearance; cast.

   The face of things a frightful image bears.
   --Dryden.

4. A representation of anything to the mind; a picture drawn by the fancy; a conception; an idea.

   Can we conceive Image of aught delightful, soft, or great?
   --Prior.

5. (Rhet.) A picture, example, or illustration, often taken from sensible objects, and used to illustrate a subject; usually, an extended metaphor.
   --Brande & C.

6. (Opt.) The figure or picture of any object formed at the focus of a lens or mirror, by rays of light from the several points of the object symmetrically refracted or reflected to corresponding points in such focus; this may be received on a screen, a photographic plate, or the retina of the eye, and viewed directly by the eye, or with an eyeglass, as in the telescope and microscope; the likeness of an object formed by reflection; as, to see one's image in a mirror.

   Electrical image. See under Electrical.

   Image breaker, one who destroys images; an iconoclast.

   Image graver, Image maker, a sculptor.

   Image worship, the worship of images as symbols; iconolatry distinguished from idolatry; the worship of images themselves.

   Image Purkinje (Physics), the image of the retinal blood vessels projected in, not merely on, that membrane.

   Virtual image (Optics), a point or system of points, on one side of a mirror or lens, which, if it existed, would emit the system of rays which actually exists on the other side of the mirror or lens.
   --Clerk Maxwell.

Virtual \Vir"tu*al\ (?; 135), a. [Cf. F. virtuel. See Virtue.]

1. Having the power of acting or of invisible efficacy without the agency of the material or sensible part; potential; energizing.

   Heat and cold have a virtual transition, without communication of substance.
   --Bacon.

   Every kind that lives, Fomented by his virtual power, and warmed.
   --Milton.

2. Being in essence or effect, not in fact; as, the virtual presence of a man in his agent or substitute.

   A thing has a virtual existence when it has all the conditions necessary to its actual existence.
   --Fleming.

   To mask by slight differences in the manners a virtual identity in the substance.
   --De Quincey.

   Principle of virtual velocities (Mech.), the law that when several forces are in equilibrium, the algebraic sum of their virtual moments is equal to zero.

   Virtual focus (Opt.), the point from which rays, having been rendered divergent by reflection of refraction, appear to issue; the point at which converging rays would meet if not reflected or refracted before they reach it.

   Virtual image. (Optics) See under Image.

   Virtual moment (of a force) (Mech.), the product of the intensity of the force multiplied by the virtual velocity of its point of application; -- sometimes called virtual work.

   Virtual velocity (Mech.), a minute hypothetical displacement, assumed in analysis to facilitate the investigation of statical problems. With respect to any given force of a number of forces holding a material system in equilibrium, it is the projection, upon the direction of the force, of a line joining its point of application with a new position of that point indefinitely near to the first, to which the point is conceived to have been moved, without disturbing the equilibrium of the system, or the connections of its parts with each other. Strictly speaking, it is not a velocity but a length.

   Virtual work. (Mech.) See Virtual moment, above.

n. a reflected optical image (as seen in a plane mirror)

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 a real location. Real images can be projected onto a diffuse reflecting screen, but a screen is not necessary for the image to form.

• 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.

• 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) or a concave 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 may be magnified or reduced depending on the position of the object.