Electron vs Optical microscopes

electron_ant
electron microscope image of ant

Scanning electron microscopes have up to 1 nm resolution  and up to 1,000,000 X magnification which is much better than optical microscopes that have up to 200 nm resolution and 1000 X magnification. That’s cool but it turns out that they’re also a bit more complicated to operate.

A month ago I wanted images of a polymer sample using a scanning electron microscope but then realized that the samples had to be specially prepared by coating them with an electrically conductive material. I didn’t have any expertise in this art so the images were never made. But, it’s interesting to figure out why such preparation is necessary.

From this website I learned that unlike an optical microscope, the electron microscope works in the following manner:

  1. The light source is replaced by a beam of very fast moving electrons.
  2. The specimen usually has to be specially prepared(i.e. conductive coating) and held inside a vacuum chamber from which the air has been pumped out (because electrons do not travel very far in air).
  3. The lenses are replaced by a series of coil-shaped electromagnets through which the electron beam travels.

The reason why the magnification allowed by an SEM is much more important than that of an optical microscope is due to the wavelength of an electron(1eV), 1.23 nm, which is much shorter than the wavelength of a photon(1eV),1240 nm. This may be computed using the relation found by De Broglie: \lambda = \frac{h}{p} where h is planck’s constant and p is the momentum.

\frac{\lambda_{photon}}{\lambda_{electron}} \approx 1000 which is the main reason why the magnification of SEM’s is approximately 1000X more important than that of optical microscopes.

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