Radiation

The word radiation comes from the Latin for "ray of light," and 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 be in the form of a spray of subatomic particles , like miniature bullets from a machine gun, or in the form of electromagnetic waves, which are nothing but pure energy and which include light itself, as well as radio waves and several other kinds.

The word radiation is also sometimes used to describe the transfer of heat from a hot object to a cooler one that it is not touching; a 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 are feeling is infrared radiation, a form of electromagnetic energy that makes molecules move faster, and therefore behave hotter, when it strikes them.

When many people hear the word "radiation," they think of the radiations that come from radioactive materials. These radiations, some of which are particles and some of which are electromagnetic waves, are harmful because they are of such high energy that they damage materials through which they pass. This is in contrast to light, for example, which has no lasting effect on, say, a pane of glass through which it passes.

The higher energies of radiation are called ionizing radiations because when they tear apart atoms they leave behind a trail of ions, or atoms that have had some of their electrons removed. Ionizing radiations include x rays , alpha particles, beta particles, and gamma rays.

Many kinds of lower-energy radiations are quite common and are harmless in reasonable amounts. They include all colors of visible light, ultraviolet and infrared light, microwaves and radio waves, including radar , TV and FM, short wave and AM. All of these radiations are electromagnetic radiations.

Electromagnetic radiation

Electromagnetic energy travels in the form of waves, moving in straight lines at a speed of 3.00 × 108 meters per second, or 186,400 mi (299,918 km) per second. That speed is usually referred to as the speed of light in a vacuum , because light is the most familiar kind of electromagnetic radiation and because light slows down a little bit when it enters a transparent substance such as glass, water , or air. The speed of light in a vacuum, the velocity of electromagnetic waves, is a fundamental constant of nature.

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 number of vibrations or cycles per second. Scientists call one cycle per second a hertz, abbreviated as Hz. Known electromagnetic radiations range in frequency from a few Hz for the lowest energies up to more than 1020 Hz for the highest.

Particulate radiation

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 , 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 accelerators, nuclear reactors, and other kinds of laboratory apparatus.

The only particulate radiations that might be encountered outside of a laboratory are the alpha and beta particles that are emitted by radioactive materials. These are charged subatomic particles: the alpha particle has an electric charge of +2 and the beta particle has a charge of +1 or -1. Because of their electric charges, these 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 altered, with unhealthful consequences.

Radiation and health

Large doses of any radiations, ionizing or not, can be dangerous. Too much sunlight, for example, can be blinding. Lasers can deliver such intense beams of light that they can burn through metal , not to mention human flesh. High levels of microwaves in ovens can cook meats and vegetables . On the other hand, as far as anyone has been able to determine, small amounts of any kind of radiation are harmless, including the ionizing radiations from radioactivity. That's just as well, because there are unavoidable, natural radioactive materials all around us.

Depending on the energy, intensity, and type of radiation, radiation may be harmful or quite harmless. It is all a matter of what kind and how much.

See also Electromagnetic field; Ionizing radiation; Nuclear medicine; Radioactive pollution; Radioactive waste.

Robert L. Wolke