The term radiation strikes fear into 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 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.

The energy from radiation sometimes increases molecular movement slightly, causing a gentle warming or a change in state, such as from solid to liquid. 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 radioactive. Instability in the atomic nuclei of radioactive substances causes them to cast off rays or subatomic particles. This process, called radioactive decay, may result in a more stable form of the same element or in a different element altogether. Often, the new elements that result from radioactive decay are also unstable and undergo further decay.