Which of the following is a lens found on electron microscopes but not on light microscopes?

While transmission electron microscopes (TEMs) continue to become more accessible thanks to novel automation, easy-to-use software, and decreasing space requirements, they can still be quite intimidating for new users who have never worked with an electron microscope before. They may be surprised to learn, however, that these instruments are not too different from the familiar light microscope.

Light microscope description

To understand how, let’s begin by considering the light microscope, shown below. In a light, or optical, microscope, a light source is placed below the sample, which must be thin enough to allow some light to pass through. (If it isn’t, the object will appear dark, and no details can be discerned.) The light is focused onto the sample with a condenser lens, resulting in a focused point of light on the specimen’s surface. As the white light interacts with the sample it is modified by its molecules, which selectively absorb or reflect only certain kinds of light. This results in the color and contrast information that makes up the image. Lenses then modify this projected information into a magnified image that we perceive with our eyes.

Diagram of a light microscope with key elements highlighted.

Electron microscope description

In an electron microscope, an electron beam takes the place of the light source. Due to the wave-particle duality of electrons in a vacuum, they can behave just like the photons that make up light. However, electrons travel as much smaller wavelengths than visible light, enabling them to reveal smaller details than what is possible with light. Instead of a glass condenser lens, electron microscopes use electromagnetic or electrostatic lenses. These produce magnetic and electric fields that guide and focus the electrons. The sample is still positioned over a focused point of the incident beam but must now be even thinner to account for the size of the electrons. (Typical TEM sample thicknesses are around ~0.1µm, depending on the material.) Just like the optical microscope, the electron beam interacts with the sample atoms, carrying away contrast and compositional information. As our eyes are incapable of perceiving electrons, it is no longer sufficient to have them serve as the detector. Instead, special electron imaging devices convert the electron signal into a greyscale image, where contrast corresponds to differences in sample thickness and density.

Side-by-side comparison of a TEM and an optical microscope, with analogous parts connected. Note that the light microscope has been inverted to more clearly parallel the TEM structure.

Summary of instrument comparison

Overall, while there are different principles controlling and guiding the incident beam for both instruments, the process of image generation is broadly the same. A beam interacts with the sample and passes through to the other side; the changes in beam energy correspond to different features in the final image. Hopefully this quick comparison has dispelled some of the initial intimidation that electron microscopes pose, empowering you to pursue the unparalleled information that electron microscopy provides.

Luckily, the latest generation of cryo-transmission electron microscopes (cryo-TEMs) includes more affordable, easier-to-use tools. In late 2020, we introduced the Thermo Scientific Tundra Cryo-TEM which includes several features that improve usability and empower anyone to use cryo-EM, even researchers who are new to the technique.

Learn more about the Tundra Cryo-TEM >>

Alex Ilitchev, PhD, is a Scientific Content Writer at Thermo Fisher Scientific.

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