Microscopic probing reveals the structure of the sample.  Conventional optical microscope employs visible light as the probing agent. For electron microscopes such as transmission electron microscopes and scanning tunneling microscopes, the probing agent is electrons.  Electron microscopes operating with electrons have shorter corresponding wavelength than visible light. As the resolution of the Rayleigh criterion is proportional to the wavelength, the minimum resolvable dimension of an electron microscope is smaller than that of an optical microscope. The resolution of optical microscope is about 200 nm while that in electron microscopes can go down to 0.1 nm resolution.