Planck Relation

The Planck relation says that a single quantum of electromagnetic radiation carries energy equal to Planck’s constant times its frequency. It works because light exchanges energy in discrete packets called photons rather than in any arbitrary amount, so higher-frequency radiation must come in more energetic quanta. This is one of the key ideas behind quantum theory and explains effects like the photoelectric effect and atomic line spectra.

Use this formula when you need the energy of a photon from its frequency, such as in questions about light, X-rays, radio waves, or atomic transitions. It is also useful whenever a problem gives wavelength first, since you can convert to frequency before finding the photon energy.

• Using the formula for a beam’s total energy instead of the energy of one photon
• Forgetting to convert wavelength to frequency before applying E = hf
• Mixing up units and using frequency in kHz or MHz without converting to Hz