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Electron and Gravitational Lenses

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Introduction

Electron and Gravitational Lenses

The electron microscope uses special lenses to focus a beam of electrons rather than light. Two types of electron lenses are used in electron microscopes: electrostatic and electromagnetic. They create electric and electromagnetic fields to both concentrate and move the beam of electrons. The magnification in magnetic electron microscopes is determined by the strength of the current passing through the electric and electromagnetic lens coils. The image is focused by changing the current through the objective lens coil.

   Massive objects in the universe can act as lenses by bending the path of light that passes near them. If a light source is behind a massive galaxy, as seen from Earth, deflected light may reach the Earth by more than one path.

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Middle

   The electron microscope requires that the electron beam be in a vacuum, because electrons cannot travel far in air at atmospheric pressure. The column and specimen chamber of the electron microscope are evacuated by pumps. Living specimens cannot be examined with an electron microscope, since they will not survive in a vacuum.  

   The magnification in magnetic electron microscopes is determined by the strength of the current passing through the electric and electromagnetic lens coils. The image is focused by changing the current through the objective lens coil. In the optical microscope the image is determined by absorption of light by the specimen; in the electron microscope the image results from a scattering of electrons by atoms of the specimen.

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Conclusion

   Scanning electron microscope.

The scanning electron microscope reveals the surface structure or topography of objects directly. Like the transmission electron microscope, it has an electron gun, condensers, and objectives. Its extremely narrow beam of focused electrons moves over, or scans, the specimen. Two types of electrons--backscattered and secondary--are emitted from the surface of the specimen. Each type has its own detector. Backscattered electrons move in straight lines, whereas secondary electrons move in curved paths. The emission of secondary electrons allows for the fine detailing of electron micrographs.  

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