Figure 1-37 Although the situation seems unrealistic, an image location can still be calculated using the lensmaker’s equation. If you try this using trial lenses, however, you will not observe an image 20 cm to the right of P′. See text for

explanation. (Illustration developed b y Edmond H. Thall, MD.)

Figure 1-38 Example of imaging by a lens with negative power. The object is real, but the image is virtual. (Illustration

developed b y Edmond H. Thall, MD.)

Figure 1-39 The first lens produces an image 12.5 cm to the right of P1′. Note that the outlines of the lenses have been

omitted. Because all distances are measured from principal points, the lens outlines provide no additional information, and

the diagram is less cluttered without them. (Illustration developed b y Edmond H. Thall, MD.)

Figure 1-40 Another 2-lens example. In this case, light never reaches the image produced by the first lens (inverted dashed arrow) because it is intercepted by the second lens. Placing a screen 50 cm behind the first lens will not show an

image. (Illustration developed b y Edmond H. Thall, MD.)

Focal Lengths

For any optical system, the distance from the anterior principal point to the anterior focal point is the anterior focal length (AFL). Similarly, the posterior focal length (PFL) is the distance from the