The Electron Microscope

Intermediate Physics for Medicine and Biology does not discuss one of the most important instruments in modern biology: the electron microscope. If I were to add a very brief introduction about the electron microscope to Intermediate Physics for Medicine and Biology, I would put it right after Sec. 14.1, The Nature of Light: Waves Versus Photons. It would look something like this.

14.1 ½ De Broglie Wavelength and the Electron Microscope

Like light, matter can have both wave and particle properties. The French physicist Louis de Broglie derived a quantum mechanical relationship between a particle’s momentum p and wavelength λ

λ = h/p     (14.6 ½)

[Eisberg and Resnick (1985)]. For example, a 100 eV electron has a speed of 5.9 × 10⁶ m s⁻¹ (about 2% the speed of light), a momentum of 5.4 × 10⁻²⁴ kg m s⁻¹, and a wavelength of 0.12 nm.

The electron microscope takes advantage of the short wavelength of electrons to produce exquisite pictures of very small objects. Diffraction limits the spatial resolution of an image to about a wavelength. For a visible light microscope, this resolution is on the order of 500 nm (Table 14.2). For the electron microscope, however, the wavelength of the electron limits the resolution. A typical electron energy used for imaging is about 100 keV, implying a wavelength much smaller than an atom (however, practical limitations often limit the resolution to about 1 nm). Table 1.2 shows that viruses appear as blurry smears in a light microscope, but can be resolved with considerable detail in an electron microscope. In 1986, Ernst Ruska shared the Nobel Prize in Physics “for his fundamental work in electron optics, and for the design of the first electron microscope.”

Electron microscopes come in two types. In a transmission electron microscope (TEM), electrons pass through a thin sample. In a scanning electron microscope (SEM), a fine beam of electrons is raster scanned across the sample and secondary electrons emitted by the surface are imaged. In both cases, the image is formed in vacuum and the electron beam is focused using a magnetic lens.

To learn more, you can watch a YouTube video about the electron microscope. Nice collections of electron microscope images can be found at http://www.denniskunkel.com, http://www5.pbrc.hawaii.edu/microangela and http://www.mos.org/sln/SEM.

Structure and function of the electron microscope. 

https://www.youtube.com/watch?v=fToTFjwUc5M