What is the constant that relates the energy of a photon to its frequency?

The constant that relates a photon’s energy to its frequency is the Planck constant.

It is represented by h and appears in the equation E = hf, where E is photon energy and f is frequency. A higher-frequency photon carries more energy, while a lower-frequency photon carries less. Because frequency and wavelength are related by the speed of light, the same idea can be written as E = hc/λ.

Max Planck introduced the constant in 1900 while studying the spectrum of black-body radiation. He proposed that energy exchange occurred in discrete amounts rather than continuously. Albert Einstein later applied this idea to light and the photoelectric effect, treating light as packets of energy now called photons.

The Planck constant has the exact SI value 6.62607015 × 10⁻³⁴ joule-seconds. Its reduced form, ħ, equals h divided by 2π and is common in quantum mechanics. A frequent mix-up is the Boltzmann constant: it connects temperature with particle energy, whereas the Planck constant connects frequency with quantum energy.