State Bethe law. Explain electrostatic focusing.
1 Answer
  • In Electrostatic Focusing, Electrostatic lens consists of three anodes, with the middle anode at a lower potential than the other two electrodes. An electrostatic focusing system is shown in figure.
  • In figure, two anodes and its electrostatic lines and equipotential surfaces are shown.
  • Potential difference is kept between these two electrodes so that an electric field is generated between them. Spreading of electric field is caused because of repulsion between electric lines.
  • If equipotential lines are drawn, as shown in figure, they would bulge at the center of the two anodes, as we know that electrons move in a direction opposite to that of electric field lines and equal-potential surfaces are perpendicular to the electric field lines so force on the electron is exerted in the direction normal to the equipotential surface.

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