What is a photon?

The fundamental particle of light is both ordinary and full of surprises. What physicists refer to as photons, other people might just call light. As quanta of light, photons are the smallest possible packets of electromagnetic energy. If you are reading this article on a screen or a page, streams of photons are carrying the images of the words to your eyes. In science, photons are used for more than just illumination. "They're ubiquitous," says Richard Ruiz, a research associate at the Institut...