photon energy

Meaning: Electromagnetic radiation comes in packets called photons. A photon's energy depends only on its frequency: higher frequency, shorter wavelength, more energy. Intensity sets how many photons arrive per second, not the energy of each. In the photoelectric effect, one photon gives its energy to one electron, which escapes only if the photon energy exceeds the metal's work function. In atoms, an absorbed or emitted photon has energy equal to the gap between two energy levels.

Equation: E = hf = hc/λ, where E is photon energy (J), h is Planck's constant, 6.63 × 10⁻³⁴ J·s, f is frequency (Hz), c is the speed of light, 3.00 × 10⁸ m/s, and λ is wavelength (m).

On the MCAT: Passages ask you to rank regions of the spectrum (radio lowest, gamma highest), to apply Kmax = hf − φ with a threshold frequency, or to link a spectral line to an electron transition, ΔE = hf. A typical calculation: halving the wavelength doubles the photon energy. Energies are often given in electron-volts, where 1 eV = 1.60 × 10⁻¹⁹ J.

Common confusion: Brighter light does not mean more energetic photons. Below the threshold frequency, even intense light ejects no electrons. Above it, more intensity gives more ejected electrons (greater current), while higher frequency gives a greater maximum kinetic energy.