photons

Meaning: Light can behave as a stream of particle-like packets called photons. Each photon has no rest mass, travels at c in a vacuum, and carries a fixed energy set only by its frequency. Intensity (brightness) reflects the number of photons per second, not the energy of each one. Atoms absorb or emit a photon when an electron moves between quantized energy levels, and the photon energy matches 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 questions that rank photons by energy (UV > visible > IR; blue > red), calculate the wavelength emitted in a transition such as in hydrogen (ΔE = hf), or apply the photoelectric effect: an electron is ejected only if hf is greater than the work function, and the maximum kinetic energy is hf − φ. Spectroscopy passages also use photon absorption (UV-Vis, IR).

Common confusion: Students think a brighter light of low frequency can eject electrons if made bright enough. In the photoelectric effect, each electron absorbs one photon, so a photon below the threshold frequency fails no matter how many arrive. Greater intensity gives more electrons (more current), not faster ones.