Light is made up of particles called photons. Photons are massless and always move at the speed of light in a vacuum. They are elementary particles and the quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are created when electrons jump between orbitals in an atom, releasing energy in the form of a tiny packet of light energy. Photons exhibit both wave-like and particle-like behavior, a concept known as wave-particle duality. They can interfere with each other to produce patterns of light and darkness, and researchers can even count photons or perform measurements on a single one. Photons are used in particle accelerators to make X-rays and ultraviolet light to study the structure of molecules and viruses and even make movies of chemical reactions.