How does a Transmission Electron Microscope work ?

A Transmission Electron Microscope (TEM) is a large piece of scientific equipment that forms detailed images (called 'micrographs', specifically 'transmission electron micrographs') of extremely small objects or areas of objects by passing a beam of electrons through a very thin slice of the area of interest.

See the main page about Transmission Electron Microscopes for more about their advantages and limitations.

Video about how transmission electron microscopy works

This short video clip shows clear pictures of a transmission electron microscope with commentary explaining in very simple terms how a transmission electron microscope works.

The above explains that a TEM consists of a cylindrical tube about 2 metres long - as shown in the initial picture of a transmission electron microscope. The tube contains a vacuum, that is - there is no air or other gas inside the part of the tube where the specimen is located. This is because molecules of gases, such as those in air, absorb electrons.

The video describes how the 'electron gun' part of the TEM works by emitting electrons from a cathode, then accelerating them through an anode, after which the electrons pass through an aperture into the vacuum tube.

As it passes down through the tube the electron beam is controlled by electromagnetic lenses formed by coils around the tube (whose effect is moderated by adjusting the electricity flowing through the coils). These electromagnetic lenses direct the electron beam through the centre of the tube to a very thin specimen located part-way down the tube.

Formation of an image of the specimen is possible because the electrons in the beam are affected by different regions of the specimen in different ways:

1. Some parts of the specimen might allow electrons to pass through unaffected.
2. Other regions within the specimen absorb some or all of the electrons that reach them - so that fewer, if any, electrons continue from that part of the specimen further down the tube to the image-formation plane, and those that do continue may do so with less energy because some of their energy has been absorbed by, or 'passed to', the part of the specimen that the electron(s) passed through.
3. Some tiny regions of the specimen may deviate the path of the electrons forming the beam so that the electrons 'bounce' off those precise positions or areas within the specimen in a range of directions. That is, instead of either continuing down the tube as in 1., or stopping (or at least slowing-down) by being absorbed as in 2., some parts of the specimen may cause the electrons to "bounce off in different directions" - that is, to scatter the electrons.

Depending on if and how they have been affected by the specimen, electrons continue down the tube (through further electromagnetic lenses) with a range of energies. They eventually form an image when they reach an image plane where they are detected by a suitable sensitive material such as a fluorescent film.

The image produced is a greyscale (not colour) image.
The darker areas represent regions of greater absorption of electrons by the specimen while lighter areas correspond to parts of the specimen that absorbed fewer, if any, electrons. A very simple way to think of this is that denser regions contain more matter so are able to absorb more energy from the electron beam whereas less dense regions such as gaps between the material forming the sample cannot absorb as much, if any, energy from the beam - hence the resulting micrograph may be thought of as a representation of which parts of the specimen absorbed relatively more or less energy from the original electron beam.

Note: The above video clip refers to the microscope only as an electron microscope, not specifically as a Transmission Electron Microscope (TEM) as opposed to a Scanning Electron Microscope (SEM). It is important to know that in a TEM the electron beam is transmitted through specimen (as above), while SEMs have scanning coils (not shown above).

See also advantages and limitations of Transmission Electron Microscopes.