Planck constant

The Planck constant is a fundamental constant of nature that relates a photon’s energy to its frequency and sets the scale of quantum effects.

Planck–Einstein relation: The equation E = hf relates a photon’s energy to its frequency through Planck’s constant. This is the constant’s direct link between light frequency and photon energy.
Max Planck: A German theoretical physicist who introduced energy quantization while studying blackbody radiation. Planck introduced the constant in 1900 to derive the spectrum of thermal radiation.
Photoelectric effect: The emission of electrons from a material when it absorbs electromagnetic radiation of sufficient frequency. Einstein’s explanation used photon energy hf to account for the frequency threshold.
Classical mechanics: The framework describing motion through Newtonian laws or related continuous theories. Classical mechanics emerges when relevant actions are large compared with h.
Reduced Planck constant: The constant ℏ equals Planck’s constant divided by 2π and is commonly used in quantum mechanics. Many quantum equations use ℏ instead of h because angular frequency is measured in radians per second.
Ultraviolet catastrophe: The classical prediction that a blackbody emits unlimited energy at high frequencies, contrary to observation. The failure of classical physics prompted Planck’s quantized energy model.
Semiconductor: A material whose electrical conductivity lies between that of a conductor and an insulator and can be controlled. Semiconductor devices depend on quantum energy levels whose scales involve h.
Quantum mechanics: The physical theory describing matter and radiation through quantized states, probabilities, and observables. The Planck constant sets the scale of this broader framework’s characteristic effects.
Compton wavelength: A particle’s Compton wavelength is the wavelength associated with its mass through Planck’s constant and the speed of light. It shows how h connects a particle’s mass to a characteristic quantum length.
Black-body radiation: Electromagnetic radiation emitted by an ideal absorber in thermal equilibrium, with a spectrum determined by temperature. Planck extracted h by fitting the observed spectrum of an idealized thermal emitter.

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