Actually, the device discussed cannot form a virtual or any other kind of image by itself. Your eye (lens) is required to form an actual image on your retina. In another type of telescope, a positive (think convex) lens (system) is used to form a virtual (in the air) image and a second positive lens system is used to focus along with your eye's lens at the virtual image. The magnification of such a system is Fo/Fe where Fo is the focal length of the main objective and Fe is the focal length of the eyepiece. Unlike a Galilean telescope, which uses a positive main objective and a negative (think concave) eye objective and forms no image, virtual or otherwise, the image appears inverted. This is common in astronomical telescopes. My rifle scope forms two virtual images using a total of three positive lens groups thereby righting the perception of the image on my retina. Remember a normal image on your retina is inverted. Regards, Bob... From: "ADavidhazy" <ANDPPH@ritvax.isc.rit.edu> > > Does anybody know why the image in an apartment door peep-hole is not > > inverted? Is there a tiny little penta-prism in there? > > The real short answer is that the image is formed by a negative lens system and > is a virtual image. Such lenses form right side up and unreversed virtual > images of subjects. These virtual images can be seen and photographed but can > not themselves make an impression on photographic film (since they are > "virtual" ... not "real" as images made by positive lenses are).
