In physics, what does the term 'radiation' refer to?

Radiation in physics refers specifically to the process by which energy is emitted as particles or waves. When considering electromagnetic radiation, it includes all forms of electromagnetic waves, which encompass visible light, radio waves, X-rays, and more. This type of radiation does not require a medium to travel through, meaning it can propagate through a vacuum, such as space.

The other options describe different physical phenomena. For example, the flow of electric current refers to the movement of electrons through a conductor, rather than energy transfer via electromagnetic waves. The movement of charged particles can refer to ionized gases or particle physics but does not encapsulate the scope of electromagnetic radiation. Lastly, conduction of heat involves the transfer of thermal energy through direct contact between materials, without involving waves or particles transmitted over a distance. Thus, the most accurate description aligning with the concept of radiation is indeed the electro-magnetic waves in space.