photon as the quantum of the electromagnetic field
/ photon = FOH-ton /
Light feels smooth and continuous, like water from a tap. But turn the tap down far enough and the stream breaks into separate drops. Light does the same: dim it enough and you find it arrives in indivisible lumps of energy. Each lump is a photon — the smallest possible amount of light of a given color. You cannot have half a photon of red light; you get one, or two, or none.
The energy of a single photon is set entirely by its frequency (color): higher-frequency light, like ultraviolet or X-rays, carries more energy per photon than lower-frequency light like red or infrared. The relation is E = hf, where f is the frequency and h is Planck's constant. This is why ultraviolet light can damage skin while equally bright red light cannot: it is not the total brightness but the energy of each individual packet that matters. Einstein used exactly this idea to explain the photoelectric effect, where light knocks electrons out of a metal only if each photon is energetic enough.
In modern particle physics the photon is more than a packet of light: it is the quantum, or smallest excitation, of the electromagnetic field that fills all of space. The same field whose ripples we see as light also carries the electric and magnetic forces, and the photon is the particle that does that carrying. The photon has no mass and no electric charge, always travels at the speed of light, and is the force carrier of electromagnetism in the Standard Model. It is the simplest and best-understood example of the deep idea that every fundamental force has its own field, and every field has its own particle.
A solar panel and a digital camera both work one photon at a time: each absorbed photon frees one electron, which is why dim ultraviolet can register where bright red light gives nothing.
Brightness is how many photons; color is how much energy each one carries.
A photon is not a tiny ball of light flying along; outside of being emitted or absorbed it is a wave-like excitation of a field, with no definite position until it interacts.