A quantum of radiation
A photon has no electric charge and zero rest mass, and travels at light speed in vacuum. The term includes quanta of radio, infrared, ultraviolet and gamma radiation, not just visible light. Invisible radiation is therefore still part of the electromagnetic spectrum.
Frequency and energy
Photon energy follows E=hf, with frequency f and Planck’s constant h. Increasing the intensity of fixed-frequency light generally increases photon arrival rate rather than individual photon energy. Higher-frequency radiation has more energetic photons and, in vacuum, a shorter wavelength.
Detection events and interference
Sensitive detectors record weak light as individual localized events. Repeated events can nevertheless accumulate into an interference pattern. Such measurements require a quantum description: localized detection does not establish that every photon followed a classical, continuously observable trajectory between source and detector.
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Exchanging energy with matter
Matter can absorb or emit photons, and the photoelectric effect demonstrates light-induced electron emission. Spectroscopy, light detectors and lasers exploit different aspects of energy exchange between electromagnetic radiation and electronic states, connecting photon physics to both measurement and the control of light.