How does an SEM work?

A scanning electron microscope (SEM) uses high-energy electrons to scan a specimen’s surface, creating a 3D image. In a conventional SEM, the specimen is placed in a vacuum. An electron beam passes through a couple of condenser lenses, which apply a magnetic field to narrow the beam and apertures (holes) to block stray electrons. The beam causes some electrons to be ejected, which can then be detected and amplified to produce an image of the sample surface.

Answered by Chi Wing Man.

  • john smith

    I disagree with this explanation. Why would sealing the straw lower the air pressure inside the straw? If I were to randomly capture a box of air from the room I am in, the air pressure in that box wouldn’t randomly drop.

    I think the answer is: The liquid can’t escape from the bottom of the straw because as the liquid drops, it creates a partial vacuum in the upper straw, (now there is more space in a region that had a fixed pressure, thus lowering the pressure), once the water level starts to drop, then the air pressure at the opening starts to become greater than the air pressure in the straw, thus a net upward force on the water.

    This is also the reason that the liquid doesn’t go upward. An upward moving liquid starts to compress the air inside the straw. That higher air pressure then pushes back on the liquid.

    Correct me if I’m wrong of course.

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