Planck's constant

Meaning: Planck's constant, h, links the energy of a single photon to the frequency of its light. Energy is exchanged in discrete packets called quanta, so a higher-frequency (shorter-wavelength) photon carries more energy. The value is h = 6.63 × 10⁻³⁴ J·s. The same constant governs the energy of photons absorbed or emitted when an electron changes levels in an atom.

Equation: E = hf = hc/λ, where E is photon energy (J), h is Planck's constant (J·s), f is frequency (Hz, or s⁻¹), c is the speed of light (3.00 × 10⁸ m/s) and λ is wavelength (m).

On the MCAT: In photoelectric effect passages, KEmax = hf − φ, where φ is the work function (J). A graph of KEmax against f is a straight line. Its slope is h, and its x-intercept is the threshold frequency. In atomic spectra questions, ΔE of an electron transition equals hf of the photon, so you compare which transition emits the shorter wavelength. Questions also use de Broglie's λ = h/p, where p is momentum (kg·m/s).

Common confusion: Students expect brighter light to give more energetic photons or faster ejected electrons. Intensity changes only the number of photons per second. Photon energy is set by frequency alone, and h is the same for every kind of light.