The classical prediction fails at short wavelengths because it says blackbody radiation should rise without limit as wavelength gets shorter. Real blackbody spectra instead reach a peak and then decline. This mismatch is the ultraviolet catastrophe; Max Planck’s 1900 proposal that energy is quantized offered a way to explain the observed spectrum.
Quiz: Why does the classical prediction fail at short wavelengths?
Answer: Classical physics predicts a runaway increase in blackbody radiation intensity at shorter wavelengths, while measured spectra have a finite peak and then fall. This failure is called the ultraviolet catastrophe. Planck proposed that radiant energy is exchanged in discrete amounts, rather than continuously, to account for the difference.
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What is the ultraviolet catastrophe?
Blackbody radiation is electromagnetic radiation emitted by an object, with a spectrum that depends on its temperature. The classical calculation predicted that the intensity would grow without limit as wavelength decreased. The term “ultraviolet catastrophe” names the failure of that prediction; it does not mean that real objects emit infinite ultraviolet energy. The observed spectrum has a peak and declines at shorter wavelengths.
The Rayleigh–Jeans law describes the long-wavelength behavior but fails at short wavelengths, as summarized in Fiveable’s College Physics I study guide, updated July 2026.
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How did Planck explain the mismatch?
In 1900, Max Planck proposed that electromagnetic energy is quantized: it is exchanged in discrete amounts rather than in an unrestricted continuum. The energy of a quantum is expressed as E = hν, where E is energy, h is Planck’s constant, and ν is frequency. Because higher frequency means a higher-energy quantum, Planck’s proposal changes the predicted distribution of emitted energy and produces a finite spectral maximum rather than the classical runaway. Chemistry LibreTexts’ lesson on quantized energy and photons introduces this relation and its connection to blackbody radiation.
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What should you remember?
- The ultraviolet catastrophe is a problem with the classical prediction, not a description of infinite radiation in nature.
- Classical physics predicts unbounded intensity as wavelength shortens; the observed spectrum peaks and then declines.
- The Rayleigh–Jeans law works at long wavelengths but not at short wavelengths.
- Planck’s quantization proposal, introduced in 1900, helped account for the observed blackbody spectrum.
- Increasing temperature shifts the spectrum’s peak toward shorter wavelengths.
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