The Nature of Light: What Is a Photon? (textbook)

A photon is a tiny, indivisible unit of light and other electromagnetic radiation. It is both a particle and a wave, exhibiting properties of both depending on how it is observed. Photons carry energy, which depends on their wavelength or color, and travel at the speed of light in a vacuum. They are responsible for phenomena like sunlight, radio waves, and X-rays. Understanding photons helps explain how energy is transferred through light and how electromagnetic radiation interacts with matter. In essence, a photon is the fundamental quantum of light, bridging the concept of waves and particles in physics.