What is 'virtual object' in optics?

Virtual object is an object that is not exist and it is just an image. It may be an image produce by certain lens or mirror. With respect to another lens or mirror, it becomes an object. But as it does not really exist, it is called the virtual object.

2006-11-04 08:46:10 補充：
No matter it is an object or image, if it is virtual, use dotted line, if it is real, use continuous line.

A real image is an image that can be projected on a surface (e.g. a screen, film). An example would be the pin hole camera, the image is inverted but it can be projected on a surface so that you can see it on a piece of paper or cardboard from any angle.

A virtual image is an image that cannot be projected on a surface. The formation of the image is just the misperception of the brain (or the eyes). An example would be the mirage - The mirage is formed by continous refraction of light from a distant object, but the brain (or the eyes) misperceives that the object is not very far from the looker.

Am I make myself clear?