Optical microscopes can only magnify objects up to 2000 times—greater magnifications do not reveal any more detail. Electron microscopes can magnify objects millions of times. The first electron microscope, which could magnify objects up to 400 times, was built in 1932 by German engineers Ernst Ruska and Max Knoll. In their instrument, the image was made not by light but by a beam of tiny subatomic particles called electrons. The images created by electron microscopes are called electron micrographs. They may be viewed on television screens using video cameras, or digitized and viewed on computer screens.

How it works

In an electron microscope, a beam of electrons does the same job as light in an optical microscope. A heated filament, usually made of tungsten, emits a stream of electrons, from an electron gun. The electrons pass through a vacuum chamber from which air has been pumped out to avoid scattering and image distortion. Electromagnetic "lenses" focus the electron beam on to the specimen. The electrons either pass through the specimen or are reflected by it. A sensor captures the transmitted or reflected electrons and displays an image, called an electron micrograph, on a computer screen. Electron microscopes can magnify objects more than a million times. 

Types of electron microscope

There are two main types of electron microscope. In a transmitting electron microscope (TEM), the beam of electrons is fired through an extremely thin slice of the specimen. TEMs magnify objects up to 2 million times. In a scanning electron microscope (SEM), a very narrow beam of electrons is fired at the surface of the specimen. The beam scans across the surface and a sensor detects the electrons bouncing off. In this way, a three-dimensional image of the specimen is built up. SEMs magnify objects up to 500,000 times.
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Scanning tunnelling electron microscope

The scanning tunnelling electron microscope (STM) was developed in 1981. It can magnify up to 100 million times, which is enough to see individual atoms. It scans a charged metal pen over the surface of a specimen, so that electrons jump or “tunnel” between the pen and the surface. This creates a “map” of high points (atoms) and low points (the gaps between them).