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Focusing in a cathode ray tube is achieved by

  • A.adjusting the potential of the control grid relative to the cathode
  • B.the aquadag coating
  • C.an electrostatic lens formed by the anodes
  • D.a permanent magnet placed round the neck of the tube

Answer: C

The shaped fields between the successive anodes act on the diverging electrons much as a glass lens acts on light, bringing them to a point at the screen. Altering the potential of the focusing anode moves that point onto the screen surface.

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