Structural addition

# [SCI] Photoelectric Effect
⏎
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.
⏎
## Overview
⏎
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.
⏎
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.
⏎
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.
⏎
## Key Figures & Recognition
⏎
- **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).
⏎
## Seminal Papers
⏎
- Einstein, A. ["Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt." *Ann. Phys.* 17 (1905)](https://doi.org/10.1002/andp.19053220607)
⏎
## What This Enables
⏎
- **[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.
⏎
# Parents
⏎
* [SCI] Blackbody Radiation & Planck's Law⏎