Note:  Do not rely on this information. It is very old.


Image, in various branches of physics, signifies strictly an objective deception of the sense of sight or of hearing. The most familiar is the reflection of objects in a mirror. Here there is an appearance of reality behind a plane sheet of burnished metal, caused by regular reflection of light coming from the actual objects in front. There are no waves of light actually proceeding from behind the mirror, and such images are termed virtual. Another instance is that of the formation of a small image of the sun, by proper focussing of a magnifying glass, on a sheet of paper. Here the waves of light actually proceed from the image, which is therefore termed real. Echoes supply examples of both real and virtual images produced by reflection of sound waves. Analogous effects are obtainable with electric and other oscillations.