In quantum physics, what constant relates a photon's energy to its frequency?

In quantum physics, the Planck constant relates a photon's energy to its frequency. The relationship is written E = hf, where E is photon energy, f is frequency, and h is the Planck constant. It shows that electromagnetic energy is exchanged in discrete quanta rather than in any arbitrary amount.

Max Planck introduced the constant in 1900 while studying the spectrum of thermal radiation. His work helped resolve the ultraviolet catastrophe predicted by classical physics. Albert Einstein later used the same idea in 1905 to explain the photoelectric effect, proposing that light behaves as packets of energy.

The Planck constant is extremely small: approximately 6.62607015 × 10⁻³⁴ joule-seconds. This exact value has been fixed in the modern International System of Units. It is often confused with the reduced Planck constant, written ħ, which equals h divided by 2π and appears frequently in quantum equations.