photon's energy
Summary
A photon's energy is the energy carried by one quantum of light, E = hf = hc/λ. It rises with frequency and falls with wavelength, so a UV photon carries more energy than a red or infrared one. Brightness changes the number of photons, not the energy of each.
Detail
Meaning: Light is delivered in discrete packets called photons. Each photon's energy depends only on its frequency, not on the beam's intensity, which sets how many photons arrive per second. When an electron changes levels in an atom, it absorbs or emits a photon whose energy equals the gap between the 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: Expect rankings across the electromagnetic spectrum (radio lowest, gamma highest), wavelength-to-energy calculations, and photoelectric-effect passages where maximum kinetic energy = hf − φ, with φ the work function (J). Below the threshold frequency no electrons are ejected, however intense the light. Raising intensity at a fixed frequency gives more ejected electrons, not faster ones. Atomic spectra questions pair emitted wavelengths with energy-level gaps.
Common confusion: Brighter light does not mean more energetic photons; only higher frequency (shorter wavelength) does. Also, E = hf is the energy per photon. For a per-mole value, multiply by Avogadro's number.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry: Atoms First
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.