The transmission electron microscope (TEM)

A transmission electron microscope (TEM) forms an image by sending a beam of electrons through a very thin specimen. Magnetic lenses focus the beam, and parts of the specimen that stop more electrons show up darker. Because electrons have a far shorter wavelength than light, a TEM can resolve much finer detail: OpenStax gives electron microscopes a resolution of about 50 picometres and magnifications of 100,000 times or more, enough to see the inside of organelles.

How a transmission electron microscope works

1. Electron gun. A heated filament releases electrons by thermionic emission.
2. Anode. A high potential difference accelerates the electrons towards a positive anode with a hole in it, forming a beam.
3. Condenser lens. A magnetic lens, a coil carrying a current, focuses the beam onto the specimen. Glass lenses cannot focus electrons.
4. Specimen. The specimen is extremely thin, 20 to 100 nm. Electrons pass through it, and denser or more heavily stained parts scatter or absorb more of them.
5. Objective and projector lenses. More magnetic lenses form a magnified image and then magnify it again.
6. Screen or detector. The electrons form the image on a screen or a detector. Areas where fewer electrons got through appear dark, so the image is black and white.

The whole column is kept under a vacuum. Electrons travel best in a vacuum, and air molecules would scatter the beam.

Why electrons give better resolution

Resolution is how close together two points can be while still appearing as two. A light microscope is limited by the wavelength of visible light. Electrons behave as waves too, with a de Broglie wavelength that gets shorter as they go faster. For an electron accelerated from rest through a potential difference V, AQA A-level Physics gives:

So raising the accelerating voltage shortens the wavelength and improves the possible resolution. OpenStax quotes electron wavelengths of about 0.005 nm, compared with hundreds of nanometres for visible light. See the de Broglie wavelength guide for more.

Put cells and organelles in order of size in this question from the Cell Biology course, then think about which microscope could show each one.

Preparing a specimen for TEM

• Very thin sections, 20 to 100 nm thick, so electrons can pass through.
• Dehydrated, because the specimen sits in a vacuum. This means TEM cannot be used on living material.
• Stained with heavy metals, which are electron dense, so some structures stop more electrons and stand out.

Preparation can damage the specimen and create artefacts: things that appear in the image but were not in the living cell. AQA A-level Biology notes that for a long time scientists had to work out which features were artefacts and which were real organelles.

TEM vs SEM

A-level Physics (AQA Turning points option) also covers the scanning tunnelling microscope (STM), which maps a surface atom by atom. Ernst Ruska shared the 1986 Nobel Prize in Physics for his work in electron optics and the design of the first electron microscope, which he built in 1933; the other half went to Gerd Binnig and Heinrich Rohrer for designing the STM.

Worked examples

Example 1: the real size from a micrograph

A virus measures 45 mm across in a transmission electron micrograph taken at × 300,000. How big is the virus?

1. Real size = image size ÷ magnification = 45 mm ÷ 300,000 = 1.5 × 10⁻⁴ mm.
2. Convert: 1.5 × 10⁻⁴ mm = 1.5 × 10⁻⁷ m = 150 nm.
3. The magnification calculator does the unit conversion for you.

Example 2: wavelength of the electrons

Electrons are accelerated from rest through 5.0 kV. Find their de Broglie wavelength. Use h = 6.63 × 10⁻³⁴ J s, m = 9.11 × 10⁻³¹ kg and e = 1.60 × 10⁻¹⁹ C.

1. 2meV = 2 × 9.11 × 10⁻³¹ × 1.60 × 10⁻¹⁹ × 5,000 = 1.458 × 10⁻⁴⁵.
2. Square root: 3.82 × 10⁻²³ kg m s⁻¹, which is the electron's momentum.
3. λ = 6.63 × 10⁻³⁴ ÷ 3.82 × 10⁻²³ = 1.7 × 10⁻¹¹ m, about 17 pm.

Example 3: the voltage for atom-sized wavelengths

AQA asks you to estimate the anode voltage needed for a wavelength about the size of an atom, 0.10 nm.

1. Rearrange λ = h ÷ √(2meV) to V = h² ÷ (2meλ²).
2. V = (6.63 × 10⁻³⁴)² ÷ (2 × 9.11 × 10⁻³¹ × 1.60 × 10⁻¹⁹ × (1.0 × 10⁻¹⁰)²) = about 150 V.
3. Real TEMs use far higher voltages, giving much shorter wavelengths, as Example 2 shows.

Common mistakes

• Saying TEM has better magnification as the reason for more detail. The key is resolution; magnifying a blurred image does not add detail.
• Saying electrons are focused with glass lenses. TEM uses magnetic lenses.
• Saying TEM gives 3D images. TEM shows thin 2D sections; SEM gives a 3D-looking surface.
• Forgetting the vacuum. It is needed because air would scatter the electrons.
• Mixing up the formula. λ = h ÷ √(2meV) uses the accelerating voltage V, not the electron's speed.

Practise with a short timed paper

Microscopy is in the first year of AQA A-level Biology (3.2.1.3), and electron microscopes are in the A-level Physics Turning points option (3.12.2.6). The daily A-level Biology: Year 1 and A-level Physics: Year 2 papers are short timed papers of 8 questions, with a new one every day. For light microscopes, see parts of a microscope.

Read next

Physics · Atoms and particlesMass of electron: 9.109 × 10⁻³¹ kg, in kg, u and MeVThe mass of an electron is 9.109 × 10⁻³¹ kg, about 1/1836 of a proton. See it in kg, u and MeV, compared with protons and neutrons, with worked examples.