Photon
A discrete packet of electromagnetic radiation carrying energy proportional to its frequency: E = hf, where h is Planck's constant (6.63 × 10⁻³⁴ J·s) and f is frequency. Photons have no rest mass and always travel at the speed of light (c = 3 × 10⁸ m·s⁻¹). The photon model explains phenomena like the photoelectric effect and emission spectra.
Formula
E = hf
Real World
Hospital X-ray machines work because X-ray photons carry enough energy (due to their high frequency) to pass through soft tissue but are absorbed by denser bone, producing the shadow images doctors use for diagnosis.
Exam Focus
Always convert eV to joules (×1.6×10⁻¹⁹) before substituting into E = hf; forgetting this loses easy marks.
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