The Transmission Electron Microscope (TEM)
An accelerated electron beam can have a wavelength thousands of times shorter than visible light. The transmission electron microscope is the instrument built to actually put that short wavelength to work, forming images by threading a beam of electrons straight through an ultra-thin sample.
Recap: why a short wavelength matters
The previous lessons established that an accelerated electron behaves as a wave with a de Broglie wavelength $\lambda=h/\sqrt{2meV}$, and that this wavelength can reach atomic scale at readily achievable accelerating voltages. The full argument for why a short wavelength is what allows fine detail to be resolved at all — and a worked estimate of the voltage needed — is covered in the later lesson Advantages Over Optical Microscopes; this lesson assumes that short-wavelength result and focuses purely on how a TEM is actually built and how it forms an image from that beam.
Firing the beam through the sample
A TEM starts with an electron gun, exactly like the ones used to generate electron diffraction patterns, accelerating electrons from rest through a high potential difference — typically a few kV up to several hundred kV in a real instrument, both to shorten the wavelength further than a modest laboratory voltage would and to give the beam enough energy to actually pass through a sample. The resulting beam is directed at the sample under study.
The defining feature of a TEM is that the sample must be extremely thin — often less than $100\,\text{nm}$ thick — thin enough that a useful fraction of the electron beam passes all the way through rather than being completely absorbed. As the beam passes through, different regions of the sample scatter and absorb electrons to different extents: denser or thicker regions of the sample scatter and absorb more electrons out of the beam, while thinner or less dense regions let more electrons pass through relatively undisturbed. The beam that emerges from the far side of the sample therefore carries a pattern of varying intensity across its cross-section, point by point, that maps directly onto the internal density and thickness structure of the sample — essentially a shadow image, built from how strongly each point of the sample scattered the beam passing through it.
The name comes directly from this mechanism: the image is formed from electrons that are transmitted through the sample, not reflected or scattered off its surface. This is exactly why the sample has to be prepared so thin — without a thin enough sample, essentially none of the beam would make it through to reach the detector at all.
Focusing the beam: electromagnetic lenses
An optical microscope focuses and magnifies light using shaped pieces of glass — lenses that bend light rays by refraction. A TEM cannot use glass lenses, because glass does nothing to steer a beam of electrons. Instead, a TEM uses electromagnetic lenses: carefully shaped magnetic fields, produced by coils of current-carrying wire arranged around the beam's path, that bend the paths of the (charged) electrons in a controllable, focusable way — directly analogous to how a glass lens bends light rays, but using magnetic forces on charge instead of refraction.
Because electrons carry charge, they respond to magnetic fields in a way that neutral photons of light never can be steered by ordinary matter — this is precisely what makes electromagnetic lenses possible at all. A series of these electromagnetic lenses, positioned along the beam's path after it has passed through the sample, focuses and progressively magnifies the transmitted beam, just as a series of glass lenses would focus and magnify light in an optical microscope.
Forming the final image
After passing through the electromagnetic lens system, the focused, magnified beam is projected onto a fluorescent screen or an electronic detector, where it produces a visible, two-dimensional image. Bright and dark regions of this image correspond directly to regions of the original sample that transmitted more or fewer electrons — dense or thick regions of the sample, having scattered more of the beam away, appear dark; thin or less dense regions, having let more of the beam through, appear bright.
Two demanding practical requirements follow directly from how a TEM works. First, samples must be prepared extremely thin — often through specialist slicing or etching techniques — since the entire imaging method depends on a substantial fraction of the beam making it all the way through. Second, the whole instrument must be operated under a high vacuum: air molecules in the beam's path would scatter the electrons long before they ever reached the sample, exactly as they would scatter the beam before it produced a clean diffraction pattern in the electron diffraction experiment.
In exchange for this demanding sample preparation and vacuum operation, TEM delivers extremely high resolution — routinely fine enough to image individual large molecules clearly, and in favourable cases, individual atoms, resolving detail many thousands of times finer than any optical microscope could ever achieve.
Key takeaways
- A TEM fires an accelerated electron beam (typically a few kV to several hundred kV) through an extremely thin sample, usually under $100\,\text{nm}$ thick.
- Denser or thicker regions of the sample scatter and absorb more of the beam; thinner or less dense regions transmit more — the transmitted beam's varying intensity maps the sample's internal structure.
- Electromagnetic lenses — shaped magnetic fields from current-carrying coils — focus and magnify the beam, playing the same role glass lenses play in an optical microscope, made possible because electrons are charged and can be steered by magnetic fields.
- The focused beam is projected onto a fluorescent screen or detector, producing an image where dark regions correspond to denser/thicker sample regions and bright regions to thinner/less dense ones.
- TEM requires extremely thin sample preparation and operation under vacuum, since air would scatter the beam before it reached the sample.
- In return, TEM achieves resolution fine enough to image individual large molecules, and in favourable cases individual atoms — far beyond any optical microscope.