blue

In physics, "blue" refers to a subatomic property related to quarks, the fundamental constituents of matter. Quarks possess a "color charge," which is analogous to electrical charge and is the source of the strong force that binds them together, mediated by particles called gluons.1

Quarks come in three "colors": red, blue, and green. These are not literal colors but a way to describe their properties within the theory of quantum chromodynamics (QCD). For a stable composite particle, such as a proton or neutron, to form, the quarks must combine in a way that results in a "white" or color-neutral state, meaning their color charges cancel out. For example, a blue quark can combine with a red and a yellow (or green) quark to form a baryon.231

In the context of light, blue is defined as the color within the visible spectrum that has wavelengths ranging from approximately 450 to 495 nanometers. This is the shortest wavelength range discernible to the human eye. The blue color of the sky is a result of Rayleigh scattering, where shorter wavelengths of light (like blue) are scattered more intensely by atmospheric particles than longer wavelengths.456