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Types of electron microscope

Electron microscopes were developed in the 1930s to enable us to look more closely at objects than is possible with a light microscope. Scientists correctly predicted that a microscope that used electrons instead of visible light as the illumination source could view objects at far higher resolution than a light microscope. This is because the wavelength of visible light is what limits the resolution of light microscopes, and the wavelength of electrons is far smaller.

Over time, specialised electron microscopes have been developed that can provide information about different aspects of an object being investigated. This means that scientists can choose the microscope that is most likely to answer their questions about their sample.

What is electron microscopy?

Electron microscopes use a beam of electrons rather than visible light to illuminate the sample. They focus the electron beam using electromagnetic coils instead of glass lenses (as a light microscope does) because electrons can’t pass through glass.

Electron microscopes enable us to look in far more detail at objects than is possible with a light microscope. Some electron microscopes can detect objects that are approximately one-twentieth of a nanometre (10-9 m) in size – they can be used to visualise objects as small as viruses, molecules or even individual atoms.

Unlike light microscopes, electron microscopes can’t be used to look directly at living things because of the special preparation that samples must undergo before they are visualised. Instead, electron microscopes aim to provide a high-resolution ‘snapshot’ of a moment in time within a living tissue.

I think the electron microscope has contributed more to science than any other scientific instrument that’s ever been invented.

- Allan Mitchell, Microscopy Otago

Specialised forms of electron microscopy

Several types of electron microscope have been developed to help investigate different aspects of a sample.

The transmission electron microscope (TEM) was the first electron microscope to be developed. It works by shooting a beam of electrons at a thin slice of a sample and detecting those electrons that make it through to the other side. The TEM lets us look in very high resolution at a thin section of a sample (and is therefore analogous to the compound light microscope). This makes it particularly good for learning about how components inside a cell, such as organelles, are structured.

Electron tomography is a form of TEM that lets us see a three-dimensional view of the cell or tissue being studied. Seeing structures in three dimensions can make it much easier to understand how they relate to each other. Electron tomography can also give two-dimensional images at higher resolution than conventional TEM.

The scanning electron microscope (SEM) lets us see the surface of three-dimensional objects in high resolution. It works by scanning the surface of an object with a focused beam of electrons and detecting electrons that are reflected from and knocked off the sample surface. At low magnifications, entire objects (such as insects) viewed on the SEM can be in focus at the same time. That’s why the SEM is so good at generating three-dimensional images of lice, flies, snowflakes and so on.

CryoSEM is a specialised form of SEM that’s good for looking at things that contain moisture (such as plants or food). In cryoSEM, samples are frozen in liquid nitrogen before being viewed. This avoids the need for the complex preparation steps that are done before conventional SEM (largely to remove water from the sample). Scientists often choose cryoSEM because it gives a more accurate image of what the sample looked like before it was prepared for microscopy.

Electron backscatter diffraction (EBSD) is used to look in detail at the structure of minerals (such as those in rocks). Rather than being microscopes in their own right, EBSD detectors are add-ons to SEMs. After the electron beam is fired at the rock, the EBSD detects electrons that have entered the rock and been scattered in all directions. The pattern of scattering can tell scientists a lot about the structure of the mineral and the orientation of crystals within it.

Nature of Science

The kind of data that scientists can collect is heavily dependent on the tools that are available to them. As microscopes have become more sophisticated, scientists have been able to view objects in greater and greater detail. In turn, they have been able to answer new kinds of questions about the objects they are studying.

Useful links

Learn more about the 1986 Nobel Prize in Physics, half of which was awarded to Ernst Ruska for the design of the first electron microscope.
www.nobelprize.org/nobel_prizes/physics/laureates/1986external link

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