Physicist Ernst Ruska and German engineer Max Knoll are credited with creating the first electron microscope in 1932. It permitted researchers greater magnification and resolution than traditional light microscopes.
The theory of the electron microscope was based on the work of Hermann von Helmholtz and Ernst Abbe, two physicists. They posited that there were wavelengths that were "smaller than light" and thereby invisible to the naked eye. This was the first theory of electrons.
Following Abbe and von Helmholtz, a doctoral candidate named Louis de Broglie developed equations that would ultimately lead to the discovery of electron waves. He theorized that every particle produces an electron wave at a particular frequency.
Using magnetic lenses that direct electrons, these electron waves were ultimately captured and then directed at objects. By directing electrons at an object and measuring the frequency at which they bounce back, an image can be formed. This led to the first electron microscope.