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Planck constant

The Planck constant relates a photon's energy to its frequency: E = hν. Since 2019 it is an exact defining constant of the SI, and its fixed value defines the kilogram. It sets the scale of all quantum phenomena, from atomic spectra to the photoelectric effect — wherever it appears, classical physics gives way to quantum behavior.

Used in

  • Photoelectric effect
  • Spectroscopy

Source: SI Brochure 9th ed. — exact defining constant