An electron microscope is an instrument that uses a focused beam of electrons instead of light to create images of extremely small objects. It can magnify up to 1,000,000 times, compared to about 1,500 times for a standard light microscope, and its resolving power is roughly 250 times greater. That difference is what allows scientists to see individual molecules, viruses, and the internal architecture of cells in sharp detail.
Ernst Ruska built the first electron microscope in 1931, and the technology has been refined dramatically since then. Today, electron microscopes are essential tools in biology, materials science, semiconductor manufacturing, and forensic investigation.
How an Electron Microscope Works
A light microscope uses visible light and glass lenses. An electron microscope replaces both. Instead of light, it fires a beam of electrons from a source called an electron gun. A strong electrical voltage (typically 3 to 5 kilovolts just for extraction) pulls electrons off a fine filament, and a second voltage accelerates them down a column at high speed. Instead of glass lenses, electromagnetic lenses create circular magnetic fields that focus and condense the electron beam as it travels toward the sample.
The entire process happens inside a vacuum chamber. Without a vacuum, the electrons would collide with air molecules and scatter before reaching the sample, ruining the image. The vacuum also prevents electrical arcing in the gun assembly and extends the life of the electron source. Different types of electron microscopes require different vacuum levels, but all of them need one.
When the electron beam hits or passes through the sample, electrons interact with the material in ways that carry information about its structure, composition, and surface shape. Detectors capture those interactions and convert them into an image on a screen.
SEM vs. TEM: The Two Main Types
The two workhorses of electron microscopy are the scanning electron microscope (SEM) and the transmission electron microscope (TEM). They work differently and produce very different kinds of images.
Scanning Electron Microscope (SEM)
An SEM scans a focused beam across the surface of a sample. Electrons bounce back from the surface (called backscattered electrons) or knock loose other electrons from the sample’s atoms (called secondary electrons). Detectors collect these signals point by point and assemble them into a three-dimensional-looking image of the surface. SEM is especially good at showing the topography and texture of a sample, and it can examine large areas, up to several hundred square centimeters in some instruments. Sample preparation is relatively straightforward, and SEM time is more affordable than TEM for most research and industrial labs.
Transmission Electron Microscope (TEM)
A TEM fires electrons through an extremely thin slice of a sample. The electrons that pass through are collected below and form an image based on how different parts of the sample scattered or absorbed the beam. TEM provides atomic-scale resolution and can reveal the internal structure of cells, crystal defects in metals, and the arrangement of proteins within a virus. The tradeoff is that samples must be sliced incredibly thin (often under 100 nanometers), preparation is more involved, and each image covers only a tiny area.
In short, SEM shows you what a surface looks like. TEM shows you what’s inside.
Cryo-Electron Microscopy
One of the most significant advances in recent decades is cryo-electron microscopy, or cryo-EM. Traditional electron microscopy requires samples to be dehydrated and sometimes coated in metal, which can distort biological structures. Cryo-EM sidesteps this by flash-freezing samples so rapidly (to around −196°C) that water molecules don’t have time to form ice crystals. The result is a sample preserved almost exactly as it exists inside a living organism.
This matters enormously for structural biology. Proteins, for example, fold into specific three-dimensional shapes that determine what they do. Older techniques like X-ray crystallography required proteins to be crystallized first, which isn’t always possible and can alter their shape. Cryo-EM images proteins in something very close to their native state. Roland Fleck, director of the Centre for Ultrastructural Imaging at King’s College London, has described it as getting “as close to the state within a functioning, living organism as possible.” The technique has been used to map the structures of drug targets, viral proteins (including the SARS-CoV-2 spike protein), and molecular machines inside cells.
What Electron Microscopes Are Used For
The applications span nearly every field that deals with small structures:
- Biology and medicine: Identifying viruses and bacteria, mapping cell ultrastructure, studying how proteins interact, and understanding disease at the molecular level.
- Materials science: Examining the crystal structure of metals and alloys, finding defects that affect strength or conductivity, and characterizing new materials like nanomaterials and thin films.
- Semiconductor manufacturing: Inspecting microchips for defects during fabrication, measuring features that are only a few nanometers wide, and performing failure analysis when chips malfunction.
- Forensics: Analyzing physical evidence such as bullet markings, gunshot residue, paint chips, and fibers at a level of detail impossible with optical microscopes.
Sample Preparation
Getting a sample ready for electron microscopy is often the most time-consuming part of the process. For biological samples going into a TEM, the typical sequence involves chemically fixing the tissue to preserve its structure, dehydrating it by replacing water with solvents, embedding it in resin, and cutting it into ultrathin slices with a specialized blade called an ultramicrotome.
For SEM, samples that don’t conduct electricity (like biological tissue, polymers, or ceramics) need a thin conductive coating. This is done with a sputter coater, which deposits a layer of metal, commonly gold-palladium or iridium, just nanometers thick onto the surface. Without this coating, electrons would build up on the sample and create bright spots that obscure the image. Facilities often maintain different coaters depending on the resolution needed: a basic gold-palladium coater for routine work and a higher-end iridium or chromium coater for high-resolution imaging.
Key Limitations
Electron microscopes are powerful, but they come with real constraints. The most fundamental is that conventional electron microscopy cannot image living specimens. The vacuum environment is lethal to living cells, and the electron beam itself deposits enough energy to damage biological material. This radiation damage gets worse as you push for higher resolution, because finer detail requires a more intense dose of electrons.
Cryogenic temperatures help significantly. Freezing samples reduces radiation damage by keeping broken molecular fragments locked in place rather than letting them drift and distort the image, which is what happens at room temperature. But even cryo-EM has limits on how much total electron dose a sample can tolerate before its structure degrades.
Thickness is another challenge. Electrons scatter heavily when passing through anything more than very thin specimens. Thicker samples produce a background “fog” of unfocused electrons that washes out contrast and makes fine details harder to resolve. This is why TEM samples must be sliced so thin.
Practical barriers matter too. Electron microscopes are expensive to buy, maintain, and operate. They require dedicated facilities with stable floors, minimal vibration, and controlled electromagnetic interference. A high-end TEM can cost several million dollars, and even routine SEM instruments represent a significant investment.
How Far the Technology Has Come
Modern electron microscopes have reached resolutions that would have seemed impossible a few decades ago. Sub-angstrom spatial resolution (less than one ten-billionth of a meter) is now achievable, meaning individual atoms can be directly imaged. On the time dimension, a research group at the University of Arizona recently won the 2025 Microscopy Today Innovation Award for developing what they call the “attomicroscope,” a TEM capable of attosecond temporal resolution. Recognized by Guinness World Records as the world’s fastest electron microscope, it can capture the motion of electrons in real time, opening a window into processes that happen on timescales shorter than a quadrillionth of a second.