Ultraviolet catastrophe
Classical physics predicted infinite UV light from a hot box. Quanta fixed it.
Classical physics predicted infinite ultraviolet light from a hot black body. Real bodies don't do that.
What was the 'ultraviolet catastrophe' in classical physics?
Planck's fix
In 1900 Planck assumed light energy comes in discrete packets, E = h·f — bigger packets at higher frequencies. High-frequency (UV) modes then need so much energy per packet that they're almost never excited, so the curve turns over. The 'catastrophe' vanishes — and quantum mechanics is born.
h ≈ 6.63 × 10⁻³⁴ J·s. Five years later Einstein used the same idea to explain the photoelectric effect.
Planck thought the energy-quantum was just a mathematical trick to make the sums work. It turned out to be how the universe actually behaves.
Planck fixed it by assuming light comes in discrete chunks (quanta) — and started quantum mechanics.