Light microscope vs electron microscope at a glance
The table below is the short version. The figures are typical values, and individual instruments vary with their optics, detectors and sample.
| Feature | Light microscope | Scanning electron microscope (SEM) | Transmission electron microscope (TEM) |
|---|---|---|---|
| Illumination | Visible light | Focused electron beam scanned over the surface | Electron beam passing through the sample |
| Lenses | Glass | Electromagnetic | Electromagnetic |
| Best resolution | About 200 nm | About 20 nm down to below 1 nm | About 0.2 nm, better with aberration correction |
| Useful magnification | Up to about 1,000 to 1,500× | Tens of thousands of times in routine work, more on high-end instruments | Beyond 1,000,000× |
| Environment | Air or liquid | Vacuum in most instruments | High vacuum |
| Live samples | Yes | No | No |
| Image | Natural colour or fluorescence | Greyscale surface relief | Greyscale internal structure |
| Preparation time | Minutes to hours | Hours to days | Days to weeks for biological tissue |
Why electrons resolve more than light
Resolution is limited by wavelength. For a light microscope, the Abbe limit is d = λ / (2 NA), where λ is the wavelength and NA is the numerical aperture of the objective. With green light at about 550 nm and a high-quality oil objective, the result is close to 0.2 µm, or 200 nm. A better lens cannot move that limit much, because NA has a practical ceiling.
Electrons behave as waves too, and their wavelength shrinks as the accelerating voltage rises. At 100 kV the relativistic electron wavelength is about 0.0037 nm, and at 300 kV it is about 0.002 nm. That is more than 100,000 times shorter than green light.
Real electron microscopes do not reach that theoretical figure, because magnetic lenses suffer from aberrations and only small apertures can be used. Even so, a transmission electron microscope can show atomic-scale detail, which no conventional light microscope can do.
Resolution and magnification are not the same thing
Magnification makes an image bigger. Resolution decides how much detail the bigger image contains. Beyond the point set by resolution, you get empty magnification: a larger picture of the same blur.
This is why a light microscope is rarely used much above 1,000 to 1,500×, even though a higher eyepiece could be fitted. The electron microscope's high magnification is useful only because its resolution keeps up.
The magnification formula used in biology exams
GCSE and A-level questions usually rely on one formula: magnification = image size ÷ actual size. Use the same units on both sides. If a cell structure measures 50 mm in a micrograph and its real size is 10 µm, convert first: 50 mm is 50,000 µm, and 50,000 ÷ 10 gives a magnification of 5,000×.
Sample preparation and live imaging
Preparation is where the two technologies differ most in daily work.
Light microscopy
A sample can go from a drop of pond water to a slide in a minute. Fixed tissue takes longer, with sectioning and staining, and fluorescence work adds labelling steps. Because the sample stays at normal pressure, cells can be watched as they divide, move or respond to a drug, sometimes for many hours.
Electron microscopy
The column runs under vacuum, so water has to be removed or frozen. For conventional work, biological samples are usually chemically fixed and dehydrated. SEM samples that do not conduct need a thin conductive coating, as described in this guide to SEM sample preparation. TEM samples must be electron transparent, typically under 100 nm thick, which means resin embedding and ultramicrotomy for tissue or thinning for materials.
Every step can introduce artefacts, and the beam itself can damage delicate material. Cryo methods reduce these problems by keeping the sample hydrated in vitrified ice, but the sample is still not alive.
Rule of thumb: if the question is about what a sample does over time, start with light. If it is about structure below 200 nm, you need electrons or another method that goes beyond the diffraction limit.
What each microscope can show in a cell
Sizes below are approximate textbook values, and they explain why some organelles appear in biology textbooks only as electron micrographs.
| Structure | Approximate size | Light microscope | Electron microscope |
|---|---|---|---|
| Animal cell | 10 to 30 µm | Clear | Clear, with internal detail |
| Nucleus | About 5 to 10 µm | Clear | Pores and envelope visible |
| Bacterium | About 1 to 5 µm | Visible, little internal detail | Cell wall and internal structure |
| Mitochondrion | About 1 µm long | Small shape only | Inner membranes and cristae |
| Virus | About 20 to 300 nm | Generally not resolved | Visible |
| Ribosome | About 20 to 30 nm | Not resolved | Visible |
Fluorescent labels can reveal where a tiny structure is under a light microscope, even if its shape stays unresolved. That difference between detecting and resolving matters when reading published images.
Cost and access
A basic teaching microscope costs a few hundred pounds or euros, while research systems with cameras, fluorescence or confocal scanning cost far more. Electron microscopes sit on a different scale. Estimates, which should be taken only as rough guides, include:
- Tabletop SEM: about $30,000 to $70,000.
- Mid-range SEM: about $100,000 to $300,000, with high-end systems at $300,000 to more than $1,000,000.
- Basic TEM: from about $100,000 to $200,000.
- High-end cryo-EM systems: more than $3 million.
The purchase price is only part of the bill. Electron microscopes need stable rooms, vacuum systems, service contracts and trained staff. Most users therefore book time at a shared facility and pay by the hour, for example about $89 per hour for instrument time.
Advantages and disadvantages
Light microscope
- Advantages: live imaging, colour, fast preparation, low cost, easy to learn, portable models available.
- Disadvantages: diffraction-limited resolution, limited depth of field at high magnification, and stains or labels are often needed for contrast.
Electron microscope
- Advantages: very high resolution, large depth of field in SEM, and access to composition and crystal structure through analytical detectors.
- Disadvantages: no live samples, slower and more demanding preparation, greyscale images, beam damage, high purchase and running costs.
Coloured electron micrographs in magazines are false-coloured afterwards. The detector records signal intensity, not colour.
How to choose the right microscope
- Define the feature size. Anything clearly above 200 nm may not need electrons at all.
- Decide whether the sample must stay alive. If it must, use light microscopy.
- Decide between surface and interior. SEM suits surfaces and fractures, TEM suits internal structure in thin samples.
- Check whether you need composition. X-ray analysis in an electron microscope gives elemental information that a standard light microscope cannot.
- Estimate sample numbers and time. Light microscopy handles many samples quickly, while TEM preparation can take days.
- Look at the budget and access. A few hours at a shared facility often beats buying an instrument.
Where the two methods meet
The boundary is not as sharp as it once was. Super-resolution fluorescence techniques such as STED and PALM, recognised by the 2014 Nobel Prize in Chemistry, work around the diffraction limit and can bring labelled structures into the tens of nanometres. They still depend on fluorescent labels, so they show the labelled molecule rather than the whole structure.
Correlative light and electron microscopy combines both views on the same sample. Fluorescence finds the rare event or labelled protein, and the electron microscope then shows its surroundings in fine detail. For many projects the most useful answer to the comparison is to use both.
The bottom line
A light microscope is the right starting point for living cells, tissue overviews, teaching and quick checks. An electron microscope is the tool for structure below the 200 nm limit, for surface detail at high magnification and for materials analysis. Start from the size of the feature, the state of the sample and the available budget, and the choice usually makes itself.