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Microscopy visualizes structures that are too small to see with the human eye, but that is about all light microscopy and electron microscopy have in common. Light microscopy uses visible light or photons to illuminate a sample, while electron microscopy uses electrons, as the name suggests. Each technique has its advantages and disadvantages, discussed below.
How Does Light Microscopy Work?
The oldest of microscopy techniques, light microscopy, uses visible light and lenses to create a small object's magnified image. Basic light microscopes can be very simple e.g., a magnifying glass, and use one or several lenses for magnification. Compound light microscopes are more complex, using a system of lenses, with one set to enlarge the other's image to achieve higher resolution and sample contrast.
Samples can be illuminated from below, or light can pass through or around the objective lens (the lens closest to the sample). Light microscopes are particularly useful in biology to observe living cells and their biological activity, the uptake of food, cell division, and movement. In vivo staining helps to examine the uptake of colored pigments by cells as images often in color.
Light microscopes are relatively low cost, and samples require very little preparation – anything from a few minutes to a few hours. This means they are widely used in microelectronics, nanophysics, biotechnology, pharmaceutic research, mineralogy, microbiology, and medical diagnostics.
However, light microscopy can only magnify structures by around 1000 to 2000 times as the wavelength of visible light limits it. Electron microscopy was designed to overcome this limitation, set by the diffraction limit of visible light.
Seeing Smaller with Electron Microscopy
Electron microscopes use a beam of accelerated electrons as a source of illumination and a shaped magnetic field to form electron-optical lens systems equivalent to the glass lenses of light microscopes.
The original electron microscopes were transmission electron microscopes (TEM), which use a high-voltage beam from an electron gun to illuminate the sample. The electrons travel through a thin section of the sample to generate a 2D cross-sectional image.
There are many variations of electron microscopes, including scanning electron microscopes (SEM) which probe a sample with a focused electron beam to create a 3D image of the surface of a sample, rather than the interior, reflection emission microscopy (REM), and scanning tunneling emission microscopy (STEM).
Electron microscopy offers a better resolution than light microscopes and can reveal the structure of smaller objects not usually seen by the latter. This is because the wavelength of electrons is up to 100,000 shorter than visible light photons. Electron microscopes have a high resolving power, 250 times that of light microscopes, and magnify objects by 100,000 to 300,000 times.
Electron microscopes are used in biology and life sciences, materials research, and industry, often in quality control or failure analysis. They are utilized to view the ultra-structures of several biological and inorganic specimens such as microorganisms, cells, large molecules, biopsy samples, metals, and crystals.
However, the technique can only be used to analyze dead or dry samples, meaning it is impossible to view living things' small structures. Consequently, sample preparation can take days and can involve chemical fixation, negative staining, cryofixation, dehydration, and sectioning.
These microscopes can also only operate under a vacuum, making them expensive to build and maintain. They are also large and require more training for effective operation.
The resulting image is in black and white/greyscale. They are often false-colored to highlight structures or aesthetic reasons, but they do not reveal any additional information.
The Better Choice?
Each microscope has advantages and disadvantages; which one you chose will depend on the investigation. Anything involving living things will mean selecting a light microscope because the electron beam's high radiation will likely kill the specimen. But if you are looking at a dry or dead sample, an electron microscope will offer much higher resolution, and therefore more detail.
References and Further Reading
Aryal, S. (2018) Differences between Light Microscope and Electron Microscope, microbiologyinfo.com https://microbiologyinfo.com/differences-between-light-microscope-and-electron-microscope/. Accessed 3 March 2021.
Microbehunter Microscopy, Electron Microscopes vs. Optical (Light) microscopes, Microbehunter Microscopy [online] http://www.microbehunter.com/electron-microscopes-vs-optical-light-microscopes/. Accessed 3 March 2021.
Microscopy, The Khan Academy [online] https://www.khanacademy.org/science/high-school-biology/hs-cells/hs-introduction-to-cells/a/microscopy. Accessed 3 March 2021.
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