Planck–Einstein Relation

The Planck–Einstein relation says that light exchanges energy in discrete packets called photons, and the amount in each packet is proportional to the light’s frequency. The constant of proportionality is Planck’s constant, which sets the quantum scale for electromagnetic radiation. This works because electromagnetic waves do not deliver energy continuously at the atomic level; in quantum theory, atoms and electrons absorb or emit whole photons whose energies depend only on frequency.

Use this relation whenever you need the energy of a photon from its frequency, or vice versa. It appears in quantum physics, spectroscopy, photoelectric-effect problems, laser physics, and calculations involving electromagnetic radiation across the spectrum.

• Using wavelength directly without first converting to frequency or using E = hc/λ instead
• Forgetting that h has units of J·s, so the result must be in joules
• Mixing up photon energy with the total energy of a beam, which depends on the number of photons as well as the energy per photon