Radiation, the emission and transmission of energy and particles of matter from atoms. Electromagnetic radiation consists entirely of energy; nuclear radiation or particle radiation consists of (1) energy and (2) various particles of matter. Visible light is an example of electromagnetic radiation. The radiation produced by radium and other radioactive elements is an example of nuclear radiation.

Radiation is found all over the universe, though in various forms such as x-rays, gamma rays, and radiation from nuclear reactors. Radiation happens whenever energy moves from one place to another. Atoms and molecules radiate to give off excess energy.

The energy of radiation is called radiant energy. (Other forms of energy are chemical, mechanical, and nuclear.) Some forms of radiant energy, such as the heat produced by infrared radiation, are evident to the senses. Special instruments are needed to detect other forms, such as that of radio waves.

Energy radiated from the sun warms Earth and provides the energy that supports life on Earth. When plants make food by photosynthesis, they convert light energy into chemical energy, which is essential to almost every kind of living thing. The same energy is locked in wood, coal, and petroleum, from which it is released by combustion to do work.