Real and Virtual Objects and Images
In analyzing the action of a (perhaps complex) sequence of refracting surfaces, we have seen that we can treat each surface in turn by considering the image formed by each surface in the sequence as the source object for the next. From the point of view of the next refracting surface in the sequence, it makes no difference whether the source is an actual source of emitted (or reflected) light or the image of such a source formed by the previous refracting surfaces in the system. We cannot tell by looking at such a beam of light whether it comes from a physical object or the image formed by the previous optics. Nevertheless, it is often helpful to keep track of whether the rays that form an image actually converge at the image location (real images), such as the image of a distant object formed by a converging lens, or only appear to diverge from the image location (virtual images), such as the image of a distant object formed by a diverging lens (see the Quick-Start Guide).
We can make a similar distinction between the apparent objects that are imaged by successive refractive surfaces. Such an apparent object from which light appears to diverge as it approaches a lens surface is considered a real object for that surface; an apparent object toward which light appears to converge as it approaches a refracting surface is a virtual object for that surface (Figure 1-16). Evidently, the distinction between “real” and “virtual” objects depends on the context—that is, the location of the lens surface which will form the next image of the object in turn.
In practice, the distinction is straightforward. An image is real if it is located on the same side of the lens as the image rays (that is, with the light traveling from left to right, to the right of the lens that forms it; strictly, to the right of the second principal plane), and virtual if it is located on the other side of the lens where the image rays do not exist. An object is real if it is located on the same side of the lens as the object rays (that is, with the light traveling from left to right, to the left of the lens that is about to form an image of it; strictly, to the left of the first principal plane), and virtual if it is on the other side of the lens where the image rays do not exist.
Figure 1-16 Ray tracing for a system of 2 convex lenses in air. The real image formed far to the right by the first lens becomes a virtual object for the second lens.
(Courtesy of Edmond H. Thall, MD.)
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.