Electron Microscope

Electron Microscope


2.1 The Problem with Optical Microscopes

By the end of the nineteenth century, the limits of magnification for the optical microscope had already been reached. In 1873, Ernst Abbe discovered that the wavelength of visible light travelling through a medium (usually air) with a refractive index of n, and converging on a spot with angle α, will make an image with diameter d. This gives the formula known as Abbe’s Diffraction Limit.

The value of nsinα is sometimes referred to as the numerical aperture, NA. The angle α is basically the angle formed based upon the distance from the lens to the object, and the value d is essentially the smallest size you can view. The closer the object is to the lens, the greater the angle will be (see Figure 1).

Figure 1: Numerical Aperture
(Technology: objectives. (n.d.). Retrieved April12, 2014, from http://research.stowers-institute.org/microscopy/external/Technology/index.htm)

This means that using the average λ for light of 500 nm and an average angle α of 45 degrees, the distance d will be about 350 nm. This is small, but when you want to look at objects smaller than cells, like DNA for example, you need a much smaller value of d. If we use electrons instead of light waves, we reduce λ to much smaller amounts. The size of λ for electrons is dependent on the energy used. At 10 keV, the electron’s λ will be .01 nm, which is 50000 times smaller than the wavelength of light. We have effectively increased our magnification limit by 50000. By adding more energy, we can reduce λ even further.

Figure 2: Electron Microscope Cutaway
(Electron microscope diagram. (n.d.). Retrieved April 12, 2014, from http://www.patana.ac.th/secondary/science/IBtopics/albums/cells/CellTheory/em3.html)

(All parts refer to Figure 2)

2.2 Electron Beam

The transmission electron microscope requires a source for electrons. An electron gun is used to produce a stream...

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