The term radiation strikes fear in the hearts of most people. Yet, all of us are constantly exposed to radiation, and for the most part, we suffer few adverse effects. In fact, some forms of radiation are critical to life. Without light and heat from the Sun, both forms of electromagnetic radiation, Earth would be a dark, cold, and lifeless place.

Radiation may be in the form of electromagnetic waves such as light and gamma rays, or energetic particles such as neutrons, protons, and electrons. Regardless of its form, all radiation carries energy and affects matter by transferring its energy to the particles in matter. This causes the atoms and molecules of the affected material to vibrate or to undergo a change in their chemical arrangement or internal state or structure. Sometimes the impact of the radiation is beneficial. Light striking plant leaves, for example, causes chemical changes that make it possible for plants to store energy that they and other organisms can use for food. The addition of energy from radiation sometimes increases molecular movement slightly, and causes warming or a change in state, from solid to liquid or liquid to gas. An example of this type of change is the melting of snow in the sunlight. In other cases, radiation's energy is powerful enough to knock the electrons out of atoms or molecules, transforming them into negatively or positively charged ions. Radiation at such a high energy level is called ionizing radiation.

Scientists call substances that spontaneously give off radiation in the form of waves or particles and undergo changes at the nuclear level, radioactive. When atomic nuclei have certain numbers of protons and neutrons, they are stable. With more or fewer neutrons, the nuclei spontaneously, although not necessarily quickly, change their state into another arrangement with lower energy by emitting particles and radiation. The emitted particles are normally neutrons or electrons and sometimes alpha particles--two neutrons and two protons bound together. The radiation may be high-energy gamma radiation, a form of electromagnetic radiation with a much shorter wavelength than visible light.