Electron microscopes require a vacuum to prevent air molecules from scattering the electron beam.

The development of the electron microscope began in 1931 when Ernst Ruska and Max Knoll at the Technical University of Berlin built the first prototype. They realized that because electrons have much shorter wavelengths than visible light, they could achieve far higher resolution. However, for this to work, the mean free path of the electrons—the average distance a particle travels before colliding with another—must be longer than the distance from the electron source to the detector.In a standard atmosphere, an electron would collide with gas molecules almost instantly, scattering the beam and creating noise. To prevent this, modern transmission electron microscopes (TEM) and scanning electron microscopes (SEM) operate at pressures as low as 10 to the power of negative 7 pascals. This level of vacuum is achieved using a combination of mechanical pumps and diffusion or ion pumps.By removing 99.9999% of air molecules, the electron beam can be precisely focused using electromagnetic lenses. This technology allows scientists to see structures at the nanometer scale, which is essential for fields like virology and semiconductor manufacturing. Ernst Ruska was eventually awarded the Nobel Prize in Physics in 1986 for his fundamental work in electron optics and the design of the first electron microscope.