The word radiation comes from a Latin term that means "ray of light." It is used in a general sense to cover all forms of energy that travel through space from one place to another as "rays." Radiation may occur in the form of a spray of subatomic particles, like miniature bullets from a machine gun, or in the form of electromagnetic waves. Subatomic particles are the basic units of matter and energy (electrons, neutrons, protons, neutrinos, and positrons), which are even smaller than atoms. Electromagnetic waves are a form of energy that includes light itself, as well as other forms of energy such as X rays, gamma rays, radio waves, and radar.
In addition, the word radiation is sometimes used to describe the transfer of heat from a hot object to a cooler one that is not touching the first object. The hot object is said to radiate heat. You can feel the heat on your face when standing near a red-hot furnace, even if there is no movement of hot air between the furnace and you. What you feel is infrared radiation, a form of electromagnetic energy that we experience as heat.
When some people hear the word radiation, they think of the radiation that comes from radioactive materials. This radiation consists of both particles and electromagnetic waves. Both forms of radiation can be harmful because they carry a great deal of energy. When they come into contact with atoms, they tend to tear the atoms apart by removing electrons from them. This damage to atoms may cause materials to undergo changes that can be harmful or damaging. For example, plastics exposed to radiation from radioactive sources can become very brittle. (This effect can be contrasted to the passage of light and some other forms of electromagnetic radiation. These forms of energy generally have no lasting effect on a material. For example, a piece of clear plastic is not damaged when light passes through it.)
High energy radiation, such as that of X rays and gamma rays, is also called ionizing radiation, a name that comes from the ability of the radiation to remove electrons from atoms. The particles left behind when electrons are removed are called ions. Ionizing radiation can cause serious damage to both living and nonliving materials.
Electromagnetic radiation travels in the form of waves moving in straight lines at a speed of about 186,282 miles (299,727 kilometers) per second. That speed is correct when electromagnetic radiation travels through a vacuum. When it passes through a transparent substance such as glass, water, or air, the speed decreases. However, the velocity of electromagnetic waves, also known as the speed of light in a vacuum, is a fundamental constant of nature. That is, it cannot be changed by humans or, presumably, by anything else. (The term velocity refers both to the speed with which an object is moving and to the direction in which it is moving.)
Electromagnetic radiation can have a variety of energies. Because it travels in the form of waves, the energies are often expressed in terms of wavelengths. The higher the energy of a wave, the shorter its wavelength. The wavelengths of known electromagnetic radiation range from less than 10 −10 centimeter for the highest energies up to millions of centimeters (tens of miles) for the lowest energies.
The energy of a wave can also be expressed by stating its frequency. The frequency of a wave is defined as the number of wave crests (or troughs; pronounced trawfs) that pass a given point per second. This is usually measured in vibrations or cycles per second. Scientists call one cycle per second a hertz, abbreviated Hz. Known electromagnetic radiations range in frequency from a few Hz for the lowest energies up to more than 10 20 Hz for the highest.
Sprays or streams of invisibly small particles are often referred to as particulate radiation because they carry energy along with them as they fly through space. They may be produced deliberately in machines, such as particle accelerators (atom-smashers), or they may be emitted spontaneously from radioactive materials. Alpha particles and beta particles are emitted by radioactive materials, while beams of electrons, protons, mesons, neutrons, ions, and even whole atoms and molecules can be produced in particle accelerators (used to study subatomic particles and other matter), nuclear reactors (used to control the energy released by nuclear reactions), and other kinds of laboratory apparatus.
The only particulate radiation that might be encountered outside of a laboratory are alpha and beta particles emitted by naturally occurring radioactive materials. Both alpha particles and beta particles are charged subatomic particles. An alpha particle is the nucleus of a helium atom. It has an electric charge of +2 and a mass of 4 atomic mass units (amu). A beta particle is an electron. It has a charge of −1 and a mass of about 0.0055 amu.
Because of their electric charges, both alpha and beta particles attract or repel electrons in the atoms of any material through which they pass, thereby ionizing those atoms. If enough of these ionized atoms happen to be parts of essential molecules in a human body, the body's chemistry can be seriously disrupted, resulting in health problems.
Large doses of any kind of radiation, ionizing or not, can be dangerous. Too much sunlight, for example, can damage a person's eyes or skin. Lasers can deliver such intense beams of light that they can burn through metal—not to mention human flesh. Microwaves in ovens are at such high levels they cook meats and vegetables.
On the other hand, small amounts of any kind of radiation are generally thought to be harmless. Even low doses of ionizing radiation from radioactive materials is probably not dangerous. The latter fact is of special importance because radioactive materials occur in small concentrations all around us.