Looking at it very simply, radiation is the sending out of waves of energy. You have known about it since you were a baby-though you didn’t know what it was. When you held your hand in front of a hot stove or radiator, or a light bulb, you felt radiant heat. When you sat in the warm sun, a type of radiation called ultraviolet rays was striking your skin.
All these are examples of electromagnetic radiation. The other major type of radiation is called radioactive radiation, and it comes from either radioactive material or nuclear reactions. In radioactive radiation, particles, as well as waves of energy, are given off.
Since electromagnetic radiation is the sending out of waves of energy, we should know something about those waves. The distance between the waves is called the wavelength. The number of waves passing a given point each second is the frequency. And when all the waves within a certain range of wavelengths are grouped together, we call them the spectrum.
The group with the shortest wavelength is the X-ray spectrum. Next comes the ultraviolet spectrum. Then comes the visible-light spectrum; we can see these waves. The waves get still longer, and we can no longer see them. This is the infrared spectrum. Even longer waves are used for radio, television, and radar.
What produces all these waves? In some cases machines are required; in others they are produced naturally. Naturally made waves come from the sun. to produce any radiation requires energy. In the case of the sun, atomic energy is produced by a reaction called fusion. In the case of X-rays, a target must be bombarded with particles.
Radioactive radiation is the process of change or decay that certain elements undergo. Such elements are radioactive. They radiate particles as the nuclei of their atoms break up.