MAGNIFICATION SYSTEMS
To see more detail in an object—a postage stamp, for example—the image needs to be magnified as shown in Figure 4.1. The easiest way to do this is with a common pocket magnifier or hand lens; this is a simple magnification system, a single lens used to form an enlarged image of an object. A similar system is used to project the image of a 35-mm slide or transparency in a lecture hall. If the screen where the focused image is projected was removed and, say, a hand lens was put in its place, a second, larger image, as shown in Figure 4.2, would be produced. This is the basic principle of all microscopy—a compound magnification system, where magnification occurs in two stages and the total magnification is the product of the magnification of the first lens and the second lens. The observer looks at the first image with a lens that produces an enlarged image called a virtual image. This is the image the eye perceives—a real, projectable image does not exist where the virtual image appears to be—and is visible only as a result of the compound magnification system. A more commonplace example of a virtual image is that seen in a mirror: Standing 2 ft away from the mirror, our image in the mirror looks as if it is standing 2 ft away from the other side of the mirror. Where a white screen or glass plate substituted for the mirror, no image would be visible. In contrast, a real image is the one that could be seen on the screen—that is, projected onto the screen.
FIGURE 4.1 To see more detail in an object, here a postage stamp, the image must be magnified. This is accomplished with a simple hand lens, which enlarges the image 10 times, or 10×.
FIGURE 4.2 In a compound magnification system, magnification occurs in two stages, and the total magnification is the product of the first lens and the second lens. So, a 10× lens and a 4× lens would produce a 40× image (10 × 4 = 40), or one that has been magnified 40 times. The observer looks at the first image with a lens that produces an enlarged image called a “virtual image.” This is the image the eye perceives and is visible only as a result of the compound magnification system.
