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Why must conventional electron microscopes operate in a vacuum?
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Colour-enhanced scanning electron micrograph (magnified 50×) of a bean aphid on a leaf.
Conventional electron microscopes must operate in a vacuum because electrons cannot travel an appreciable distance in air at atmospheric pressure.1
If the column were not evacuated by pumps, the electron beam would collide with air molecules, preventing it from reaching the specimen and forming an image. To maintain this environment, specimens and other necessary apparatus are introduced into the microscope through air locks.1