Blackbody Cavity Radiation
A small hole in a heated cavity is the closest thing to a perfect blackbody, and it is where quantum theory began. This simulation shows radiation bouncing inside the cavity and compares the measured spectrum with the classical prediction that failed.
How to use this simulation
- Set the cavity temperature
- Overlay the classical Rayleigh–Jeans prediction
- Follow individual radiation modes inside the cavity
What to look for
- The classical curve diverges at short wavelengths while the real one turns over
- Planck's quantised model matches the measured spectrum exactly
- Any material lining the cavity gives the same spectrum at the same temperature
The physics behind it
Classical physics predicted intensity rising without limit at short wavelengths — the ultraviolet catastrophe. Planck resolved it by assuming energy is exchanged only in quanta of size hf, which suppresses the high-frequency modes and produces the observed curve. This assumption, introduced reluctantly, was the first quantum hypothesis in physics.