The human eye and the magnifying glass are examples of optical systems.
In everyday life, the words object and image seem clear to us. However, transforming them into a geometric model is not immediate. For example, when we take a picture of a tree on a slide film, it is clear that what is recorded on the film is an image of the object, in this case, the tree; when we project the slide onto a screen, what is on the film becomes an object, since the image is what we see on the screen.
To avoid misunderstandings, it is necessary to establish definitions that leave no room for ambiguity.
An object point is defined , in relation to an optical system, as a point determined by the intersection of incident rays in that optical system. An image point , in relation to a given optical system, is, by definition, the intersection of light rays emerging from the system.
the blue rectangle represents the optical system (S) of reference (mirror, lens, flashlight, eye, etc.). Both the object point and the image point can be real (determined by the crossing of the rays themselves) or virtual (determined by the crossing of the extensions of the rays), in relation to each optical system under study.
When the rays are parallel, or very approximately parallel (meaning that their intersection either does not exist or is too far apart, for all practical purposes), the point is said to be improper . Extended objects and images are collections of object points and image points, respectively.