Photoelectric effect
Hertz first observes light affecting electrical-discharge sparks
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Original prompt & settings (JSON) ↗Hertz covered a spark so he could see it better — and the cover made the spark itself smaller.
Karlsruhe, 25 January 1887
While investigating electrical oscillations at the Karlsruhe Polytechnic, Heinrich Hertz recorded on 25 January 1887 that light was affecting his apparatus's discharge sparks.
His radio-wave receiver was a curved conductor with a small spark gap. He was trying to see extremely small sparks; shielding the gap changed the spark rather than merely making it easier to observe.
Glass suppressed it, quartz brought it back
Two days later he was experimenting with electric light, and he carried on systematic tests through the spring.
In the investigation that followed, shielding the receiver with glass reduced the receiving spark, while quartz — which transmits ultraviolet light — restored the spark's size, pointing Hertz past visible light into the ultraviolet. He sent “On an effect of ultra-violet light upon the electrical discharge” to Annalen der Physik und Chemie on 27 May 1887.
Not what it is now called
Naming this “the photoelectric effect” is retrospective. Hertz reported a puzzling ultraviolet influence on electrical discharge and explicitly offered no electron or quantum explanation; electrons had not yet been identified.
The ultraviolet effect did not generate the radio waves either. It altered how readily a spark formed, and so changed the sensitivity of Hertz's spark-gap observations.
Sources
Researched 23 Aug 2026 6 sources 2 audit passes Date corrected
How this was checked
Researched from the web into a fact sheet, rewritten from that sheet, then audited against it by a different model. How the pipeline works →
Date corrected. The research places this at 1887-01-25.
What the sources leave uncertain
- Calling the observation “the photoelectric effect” is retrospective. Hertz reported an ultraviolet influence on electrical discharge and explicitly offered no electron or quantum explanation.
- The January 25 date is supported by the Heinrich Hertz Institute’s research chronology; the glass, quartz and prism tests belong to the investigation that followed, not necessarily to that single day.
Checked against
- The most important experiments of Heinrich Hertz — Fraunhofer Heinrich Hertz Institute
- Hertz’s photoelectric effect — Institute of Physics
- Hertz, “Ueber einen Einfluss des ultravioletten Lichtes auf die electrische Entladung” (1887)
- Photoelectric Effect — University of Virginia Physics
- Einstein and the Photoelectric Effect — American Physical Society
- Guglielmo Marconi — Nobel Prize biography