Experiment #50 · Scientific experiment

The Photoelectric Effect

Light is quantised

Albert Einstein (theory, 1905); R. A. Millikan (confirmation, 1916) · 1905 / 1916 · Quantum mechanics

First published: A. Einstein, "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt", *Annalen der Physik* 17 (1905): 132–148; R. A. Millikan, "A Direct Photoelectric Determination of Planck's 'h'", *Phys. Rev.* 7 (1916): 355–388.

Light below a threshold frequency ejects no electrons no matter how bright; above it, ejection is instantaneous. Light is delivered in quanta.

When light shines on a metal surface, electrons are emitted if and only if the light's frequency exceeds a metal-specific threshold. Classical wave theory predicts emission at any frequency given sufficient intensity, with a time delay while energy accumulates. Observation contradicted both predictions: sub-threshold light produces no emission however intense, and super-threshold light produces immediate emission however dim. Einstein's 1905 paper proposed the explanation: light is quantised into discrete "energy packets" (later called photons) of energy E = hν, and each electron interacts with one photon. Millikan, attempting to refute Einstein, instead provided definitive confirmation in 1916 — and a precise measurement of Planck's constant *h*. The result was the first decisive evidence that quantisation extends to light itself, not just to atomic transitions.

Formulation

Light of frequency ν → metal surface; measure electron kinetic energy K. Classical prediction: K depends on intensity. Observed: K = hν − W (work function), independent of intensity; emission only if ν > W/h. Conclusion: light energy comes in quanta of size hν.

Compare this experiment

Dimensions Engaged

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Matter

Forces Matter · Ontological Status to include discrete quanta of electromagnetic radiation — photons — that behave more like particles than waves in absorption.

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Energy

Establishes Energy · Granularity at fundamental scales: electromagnetic energy is exchanged in discrete units, not continuously.

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Observer

Bears indirectly on Observer · Knowledge Extent: the classical wave description, however refined, cannot capture the empirical pattern. Observation forces conceptual revision.

Responses — How Schools Engage

Affirms / takes the bait 6

Quantum Realism

Photons are real discrete entities. The experiment is the first decisive evidence that quantisation extends beyond atomic energy levels to the electromagnetic field itself.

Naturalism

A canonical empirical revision: classical theory predicted wrong; quantum theory predicted right. Einstein's heuristic of "light quanta" is empirically mandated.

Structuralism

Photons are structural: defined by their conserved quantities (energy hν, momentum h/λ) and their role in field-matter interactions, not by any intrinsic corpuscular nature.

Realism

Scientific realism: photons exist. The threshold behaviour and frequency dependence are inexplicable without granting their reality.

Logical Positivism

A model of operational confirmation: the K = hν − W relation is directly measurable, *h* is extracted as a fundamental constant. Quantum theory earns its authority through prediction, not metaphysics.

Pythagoreanism

Another confirmation of nature's discreteness: energy comes in integer-multiple packets, not as a continuum. Number is fundamental to physical reality.