# [SCI] Photoelectric Effect
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The **Photoelectric Effect** is the emission of electrons from a material when struck by light — explained by Einstein (1905) by proposing that light consists of discrete quanta (photons) of energy E = hν.
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## Overview
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When light of sufficiently high frequency strikes a metal surface, electrons are ejected. The maximum kinetic energy of ejected electrons depends on frequency, not intensity — a result impossible to explain with classical wave theory. Einstein's photon hypothesis explained it perfectly: each electron absorbs one photon of energy hν; if hν exceeds the work function φ, the electron escapes with kinetic energy hν − φ.
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This was the first direct evidence for quantisation of electromagnetic energy (Planck had quantised the emission process, not the field itself). It established the wave-particle duality of light and was the key experimental foundation for quantum mechanics.
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The photovoltaic effect in semiconductors is the solid-state version: photons promote electrons across the band gap, generating a current in a p-n junction. Every solar cell operates on this principle.
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## Key Figures & Recognition
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- **Albert Einstein** (1879–1955): Explanation of the photoelectric effect, 1905. **Nobel Prize in Physics 1921** — awarded specifically for this discovery, not special relativity.
- **Heinrich Hertz** (1857–1894): First observed the effect (1887) in EM wave experiments.
- **Philipp Lenard** (1862–1947): Quantitative measurements. **Nobel Prize 1905** (for cathode ray work; his priority dispute with Einstein was bitter).
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## Seminal Papers
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- Einstein, A. ["Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt." *Ann. Phys.* 17 (1905)](https://doi.org/10.1002/andp.19053220607)
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## What This Enables
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- **[SCI] Quantum Mechanics** — Einstein's photon hypothesis was a co-founding step of QM alongside Planck's blackbody quantisation.
- **[TECH] Solar Cells (Photovoltaics)** — The photovoltaic effect in semiconductor p-n junctions is the solid-state realisation of the photoelectric effect.
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# Parents
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* [SCI] Blackbody Radiation & Planck's Law⏎