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

# Parents

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