22.7 The Thin-Lens Equation Optics Demonstrations-Two Lenses with a Real and a Virtual Image Explanation: The video shows a system of two convergent lenses. An object is placed outside the focal point of the first lens. As we know, a real inverted image on the other side of the lens should be formed. However, the second lens is in the way, and this real image is not formed. Instead, this image becomes a virtual object of the second lens.  The image of this virtual object is an inverted upright image on the same side of the second lens as the virtual object, and inside the focal length of the second lens. To understand the behavior of this second lens, recall the behavior of a single convergent lens with the object placed inside the focal length; the behavior of the second lens is the symmetrical equivalent. The object used in this simulation is the letter T. Each lens is represented by a blue circle with dots placed on either side to designate the focal point. The moving gray square simulates a viewing screen.   Source: Samantha Shoemake and John T. Foley The Optics Project (TOP) Department of Physics and Astronomy Mississippi State University, Mississippi State, Mississippi 39762.