Planck constant: the quantum of action

planckConstant, h, relates a photon’s energy to its frequency through E = hν. Its SI unit is joule second, and its exact defined value underpins the modern SI kilogram.

Practical use

Use h to convert frequency into photon energy, often together with speedOfLight for wavelength calculations. reducedPlanckConstant is h divided by 2π and is common in angular-frequency formulas.

Caveats

Distinguish frequency ν from angular frequency ω; the latter normally uses reduced Planck constant. Use multiply for unit-consistent products and elementaryCharge when converting energy in joules to electron-volts.