Definition
The smallest indivisible unit, or quantum, of electromagnetic energy. A photon has no mass and no electric charge, travels at the speed of light, and behaves as both a particle and a wave. The energy carried by a photon is proportional to the frequency of its associated electromagnetic radiation.
Plain English
A photon is a tiny packet of light energy. Light, radio waves, and similar radiation are made up of these packets. They have no weight, carry no charge, and always move at the speed of light.
Context Anchor
Seen in explanations of light, vision, aircraft light sensors, displays, cameras, and radiation.
Derivation
From the Greek 'phos,' meaning light, with the suffix '-on' used in physics to denote a fundamental particle (as in electron, proton). The name reflects that it was first identified as the basic unit of visible light before being recognized as the unit of all electromagnetic energy.
Why Pilots Care
Pilots do not usually use the word in normal cockpit calls, but it helps explain how light reaches the eyes, cameras, and sensors used in aviation equipment.
Analogy
Think of light as falling rain. The photon is a single drop. You can't have half a drop -- it's the smallest piece the rain comes in.
Intuition Check
A photon is not a lamp or a whole beam of light. It is one tiny packet of the energy that makes up light.
Example Sentence 1
A photoelectric cell works by converting incoming photons into a small electrical current.
Example Sentence 2
Fiber optic sensors use photons traveling through cables to monitor engine parameters.