Originally posted by sucahyo
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Wikipedia:
Individual photons can be detected by several methods. In the classic photomultiplier tube (1887 Heinrich Hertz), a photon landing on a metal plate ejects an electron, initiating an ever-amplifying avalanche of electrons. Microchips use a similar effect in semiconductors: an incident photon generates a charge on a microscopic capacitor that can be detected.
Other detectors such as Geiger counters use the ability of photons to ionize gas molecules, causing a detectable change in conductivity.
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