Radiation has two meanings in physics. The first refers to propagating energy, and sometimes to the energy itself. An example is a light wave--its photons each have energy. The second meaning is the process of a body emitting energy, as in a pipe filled with hot water that radiates heat.

Radiation (in the first sense above) comes in several forms: electromagnetic radiation, acoustic radiation, and particle radiation.

Electromagnetic radiation is the radiation composed of electromagnetic waves, or, from another point of view, photons. In the relatively small band of wavelengths in which our eyes can see, we call it light. At longer wavelengths, beyond our eyes’ capabilities, we call it infrared radiation, and beyond that microwave radiation, then short radio waves, radio waves, and long radio waves. At shorter wavelengths we refer to light as ultraviolet radiation, x-rays, and gamma rays.

Acoustic radiation, or waves of sound, is...