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A photoconductive cell such as one of cadmium sulphide responds to light by

  • A.a change in the capacitance between its two electrodes
  • B.a fall in its resistance
  • C.generating a voltage across its terminals without any supply
  • D.emitting electrons from its illuminated surface into a vacuum

Answer: B

Photons free carriers within the material, so its conductivity rises with illumination. It is a passive device requiring a supply, and it is slow to respond compared with a photodiode, which limits it to lighting control and similar undemanding work.

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