BIOLOGY Volume 1 - A Guide to General Biology - 2004

5. CELLS

5.6. Electron microscopy

5.6.2. Resolving power and magnification

Resolving power, or resolution, is the ability of a microscope to distinguish between two separate points that are very close together. Below the limit of resolution, these objects appear as a single object. Resolving power and magnification are not the same thing. No matter how much we enlarge a photograph, we will never be able to resolve individual atoms on it. Magnification increases, but the resolution of the photograph remains the same. We enlarge a photograph to see the image more clearly, but if we continue to magnify it too far, the image eventually breaks down into a blur of individual pixels or patches. The difference between magnification and resolution can also be understood by comparing images of a Cell obtained using a Light Microscope and an Electron microscope at the same magnification (Fig. 5.5). As clearly seen in Fig. 5.5, these two images differ significantly in their level of detail. The resolution of an electron microscope is much higher because the wavelength of electrons is much shorter than that of visible light.

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Fig. 5.5. Micrographs of the same plant Cells taken with a light microscope (A) and an electron microscope (B) at the same magnification (approx. x2500).

The practical limit of resolution for an electron microscope is about 0,5 nm, whereas for a light microscope it is 200 nm. This does not mean that the electron microscope is simply "better". These two instruments serve different purposes. The light microscope remains indispensable for getting An Overview of Cells and Tissues. Furthermore, specimen preparation for it is much faster and simpler. It also allows the observation of living specimens, which is entirely impossible with an electron microscope.