THE LENS
Most people are familiar with the lenses in our daily lives: eyeglasses, reading magnifiers, and the like. In microscopy, a lens means a very specific thing: a translucent material that bends light in a known and predictable manner. For example, an ideal converging lens causes all light entering the lens from one side of the lens to meet again at a point on the other side of the lens, as shown in Figure 4.3. In doing so, an image of the original object is produced. The size and position of an image produced by a lens can be determined through geometry based on the focal length of the lens, which is the distance between the two points of focus on either side of the lens, as shown in Figure 4.4. Focal length is important in microscopy because it determines much of the image quality. Think of it this way: If one eye is too far or too close, it is difficult to see a clear image of an object. Why? Human
eyes, being curved, cannot maintain a clear point of focus for all distances: About 10 in or 25 cm is the distance that a human eye can easily distinguish between two objects next to each other. Lenses are made using this “ideal” viewing distance, or focal length. At 25 cm, the resolution, or the minimum distance two objects can be separated and still be seen as two objects, of the human eye is between 0.15 and 0.30 mm. There fore, this is the limit of human eyes without assistance; if better resolution is required (that is, to see more detail in the postage stamp), the image must be magnified. If a hand lens magnifies an image 4 times (the shorthand for this is “4×”), then the viewer will be able to resolve two objects that are about 0.05 mm apart (for the math used to obtain this value, see “In More Detail: Why Resolution Is More Important Than Magnification”). Magnification with one lens cannot continue indefinitely, however. As magnification increases, lens diameter decreases to bend the light more to make a larger image. A simple lens that magnifies 1000× would be only 0.12 mm in diameter! Therefore, about 10× to 15× is the practical limit of magnification for simple lenses.
FIGURE 4.3 An ideal converging lens causes light entering it from one side to meet again (converge) at a point on the other side of the lens.
FIGURE 4.4 The focal length is the distance from the two focal points (F1 and F2) of a lens. F2 sits on a plane where the image will appear to be in focus when the object being viewed is at F1.
FIGURE 4.5 Empty magnification results from an increase in magnification without an increase in resolution.
