BRAINCAKEScanning Electron Microscope
In a scanning electron microscope (SEM), a fine beam of electrons is scanned back and forth across the surface of a specimen. The electrons do not pass through it. Instead, electrons are knocked off or bounced back from the surface, and a detector collects them to build up an image point by point.
Preparing the specimen
The specimen is fixed and dried, then coated with a very thin layer of a heavy metal such as gold. The metal coating reflects electrons well. Because electrons do not need to pass through, the specimen does not need to be cut into thin sections, so whole organisms such as insects, pollen grains or bacteria can be viewed.
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Key terms in this lesson
- artefact
- A structure seen in a microscope image that was created by the preparation process and was not present in the living specimen.
- depth of field
- The range of distance over which an image stays in sharp focus; a large depth of field gives a three-dimensional appearance.
More in Eukaryotic Cells
- Magnification and Resolution
- Light Microscope
- Electron Microscope
- Transmission Electron Microscope
- Ultrastructure
- Membranes
- Phospholipid Bilayer
- Membrane Proteins
All 30 lessons in Eukaryotic Cells · All Edexcel AS-level Biology topics