# [SCI] Blackbody Radiation & Planck's Law

**Blackbody Radiation** refers to the electromagnetic radiation emitted by an idealised body in thermal equilibrium. Max Planck's (1900) derivation of the correct spectral distribution required the revolutionary hypothesis that energy is quantised.

## Overview

Classical electromagnetism predicted the Rayleigh–Jeans law for blackbody emission, which diverged at high frequencies (the "ultraviolet catastrophe"). Planck (1900) fitted the observed spectrum empirically and derived the Planck distribution by assuming energy could only be exchanged in discrete quanta E = hν. He initially viewed this as a mathematical trick, but Einstein (1905) took it seriously, proposing that light itself consists of photons and explaining the photoelectric effect — launching quantum theory.

Planck's constant h = 6.626×10⁻³⁴ J·s is now a fundamental constant, defining the scale of quantum phenomena. Modern applications include thermal radiation sensors, infrared cameras, cosmological measurements (CMB), and semiconductor devices.

## Key Figures & Recognition

- **Max Planck** (1858–1947): **Nobel Prize in Physics 1918** "in recognition of the services he rendered to the advancement of Physics by his discovery of energy quanta."
- **Albert Einstein** (1879–1955): Photoelectric effect, photon hypothesis. **Nobel Prize 1921**.

## Seminal Papers

- Planck, M. "Zur Theorie des Gesetzes der Energieverteilung im Normalspektrum." *Verh. Dtsch. Phys. Ges.* 2 (1900).
- Einstein, A. "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt." *Ann. Phys.* 17 (1905).

## What This Enables

- **[SCI] Quantum Mechanics** — Planck's quantum hypothesis (E = hν) was the single seed from which all of quantum mechanics grew.
- **[SCI] Atomic Structure & Spectroscopy** — Blackbody-like analysis of atomic emission spectra revealed discrete energy levels requiring a quantum model.
- **[SCI] Photoelectric Effect** — Planck's quantisation of emission directly inspired Einstein's 1905 photon hypothesis to explain the photoelectric effect.

## Discovery Character
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**Surprise level**: Extreme — Planck described his own hypothesis as "an act of desperation." He introduced energy quantisation (E = nhν) as a mathematical trick to fix the ultraviolet catastrophe, explicitly stating he did not believe it was physically real. It turned out to be the most fundamental feature of nature.
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**Mode**: Serendipitous-of-implication. The work was systematic (fitting a curve to blackbody data), but the meaning — that energy comes in discrete packets — was neither intended nor believed by its author. Einstein later took it seriously and won the Nobel Prize for doing so. Planck's conservatism about his own discovery may be the most consequential case of a physicist not believing what his own mathematics was telling him.
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# Parents

* [SCI] Statistical Mechanics
* [SCI] Classical Thermodynamics
* [SCI] Statistical Mechanics