Photon
Definition
The smallest possible packet of light — a single particle of electromagnetic energy. When sunlight warms your skin, it is actually a stream of countless photons arriving one by one.
Symbol: · Unit: joule (J) for its energy
Technical definition
The quantum of the electromagnetic field: a massless, chargeless elementary particle that carries electromagnetic energy and momentum. Photons travel at the speed of light in vacuum, and each carries energy E = hf, where h is Planck's constant and f is the frequency.
Example
A green laser pointer emits photons with a wavelength near 532 nanometers. Each photon carries about 3.7 × 10^-19 joules — far too little to feel alone, but trillions of them per second make the bright dot you see on the wall.
Students often picture a photon as a tiny glowing ball flying through space. A photon is not a little marble; it is a quantum of a field. It travels like a wave and is absorbed like a particle — thinking of it as only one or the other leads to wrong predictions.