Which term describes a single quantum of light energy?

• A. Neutrino
• B. Electron
• C. Photon
• D. Quark

Answer: (C)

Light energy is quantized into indivisible packets called photons. Each photon is the quantum of the electromagnetic field and carries energy proportional to its frequency (E = h f) or inversely to its wavelength (E = h c / λ). This particle-like description of light explains behaviors like the photoelectric effect, where energy transfer occurs in discrete units. The other terms refer to different particles: a neutrino is a nearly massless lepton, an electron is a charged lepton with mass, and a quark is a fundamental constituent of protons and neutrons. None describe a quantum of light energy. So the single quantum of light energy is a photon.