TABLE OF CONTENTS Microscopy involves the study of objects that are too small to be examined by the unaided eye. In the SI (metric) system of units, the sizes of these objects are expressed in terms of sub-multiples of the meter, such as the micrometer (1 ?m = 10 -6 m, also called a micron) and also the nanometer (1 nm = 10 -9 m). Older books use the Angstrom unit (1 = 10 -10 m), not an official SI unit but convenient for specifying the distance between atoms in a solid, which is generally in the range 2 3 .
To describe the wavelength of fast-moving electrons or their behavior inside an atom, we need even smaller units. Later in this book, we will make use of the picometer (1 pm = 10 -12 m).
The diameters of several small objects of scientific or general interest are listed in Table 1-1, together with their approximate dimensions.
| Object | Typical diameter D | M* = 75 ?m/ D |
|---|---|---|
| Grain of sand | 1 mm = 1000 ?m | None |
| Human hair | 150 ?m | None |
| Red blood cell | 10 ?m | 7.5 |
| Bacterium | 1 ?m | 75 |
| Virus | 20 nm | 4000 |
| DNA molecule | 2 nm | 40,000 |
| Uranium atom | 0.2 nm = 200 pm | 400,000 |
Our concepts...
