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6.4.1 Planck and the Blackbody

By the end of this lesson you can:

  • Describe the ultraviolet catastrophe
  • Explain Planck's quantum hypothesis
  • Use E = hf to calculate photon energy
Find your course: NSW Year 11NSW Year 12: Module 7, IQ3QLD Unit 4: Quantum theoryVIC Unit 4: Light, matter and relativityAP Physics 2: Unit 15IB: E.2 Quantum physics
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Learn the idea: a short explanation, then a check.
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In 1900, Max Planck, in trying to understand the mathematical principles behind blackbody radiation, postulated that energy was not continuous, but came in discrete amounts.

He admitted that this was a mathematical trick to make the model fit experimental observations.

Nonetheless, his idea of energy being quantised was taken up by Albert Einstein to explain the photoelectric effect, and later by Niels Bohr to understand the structure of the atom.

What began as a "mathematical trick" led to a complete paradigm shift, giving birth to what is now known as quantum mechanics.

The following lessons explore this development, tracing the journey from Max Planck to Erwin Schrödinger and Werner Heisenberg.

Planck and Black Bodies 

In 1900, German physicist Max Planck revolutionized our understanding of light and matter while attempting to solve a major breakdown in classical physics: blackbody radiation.

The Classical Problem: The Ultraviolet Catastrophe

A blackbody is an idealized physical body that absorbs all electromagnetic radiation falling upon it. When heated, it emits thermal radiation across a continuous spectrum of wavelengths, with a peak intensity that depends solely on its absolute temperature (T in Kelvin).

Classical physics, which treated electromagnetic radiation as continuous waves, produced the Rayleigh-Jeans Law. While this law accurately predicted radiation intensity at long wavelengths (infrared), it failed spectacularly at short wavelengths (ultraviolet). Classical theory predicted that an ideal blackbody would emit infinite energy in the ultraviolet spectrum—an impossible outcome known as the Ultraviolet Catastrophe.

Additionally, empirical measurements showed that as an object's temperature increases:

  1. Total radiated energy increases dramatically (proportional to T4, described by the Stefan-Boltzmann Law).
  2. The peak emission wavelength (λmax) shifts to shorter wavelengths (higher frequencies), governed by Wien’s Displacement Law:
λmax T = b   (where b ≈ 2.898 × 10−3 m·K)

Planck's Quantum Hypothesis

To resolve the conflict between classical theory and experimental observations, Max Planck proposed a radical idea: the atomic oscillators in the cavity walls of a blackbody do not absorb or emit energy continuously. Instead, energy is exchanged only in discrete packets called quanta.

The energy (E) of each quantum is directly proportional to the frequency (f) of the radiation:

E = h f
  • E = Energy of a photon or quantum (Joules or eV)
  • h = Planck’s constant (≈ 6.626 × 10−34 J·s)
  • f = Frequency of the electromagnetic wave (Hz)

Planck initially regarded quantization as a convenient "mathematical trick" to make his theoretical model match experimental data. However, his work marked the birth of quantum mechanics—a paradigm shift later expanded by Albert Einstein to explain the photoelectric effect and by Niels Bohr to model the hydrogen atom.


This video covers the basics of the back body curve, and Planck's attempt to reconcile the observations with the current theories. His attempt led to the birth of quantum physics

Check Your Understanding: Blackbody Radiation & Planck's Hypothesis

1. What was the central failure of classical physics regarding blackbody radiation (known as the “ultraviolet catastrophe”)?
2. How did Max Planck resolve the conflict between theoretical models and the experimental blackbody radiation curves?
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Test your predictions in a simulation.

We previously encountered this curve during our discussion on blackbodies in Thermodynamics.

The fundamental issue in classical physics was not the curve's experimental shape, but rather the total inability of classical theories—which treated energy as a continuous spectrum—to mathematically model or explain why the curve took this form.