Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.


Radiation, in physics, is the process of transmitting energy through space. Such radiation can consist of waves or particles. Waves and particles have many characteristics in common; usually, however, the radiation is predominantly in one form or the other. Mechanical radiation consists of waves, such as sound waves, that are transmitted only through matter. Electromagnetic radiation is independent of matter for its propagation; speed, amount, and direction of energy, however, are influenced by the presence of matter. This radiation occurs in a wide variety of energies, with visible light about in the middle of the range. Electromagnetic radiation carrying sufficient energy to bring about changes in atoms that it strikes is called ionizing radiation. Particle radiation also can be ionizing if it carries enough energy. Like electromagnetic radiation, which it resembles, it does not require matter for its propagation. Examples of particle radiation are cosmic rays, alpha rays, and beta rays. Cosmic rays are streams of positively charged nuclei, mainly those of hydrogen. Cosmic rays may also contain electrons, protons, gamma rays, pions, and muons. Alpha rays are streams of positively charged helium nuclei. Beta rays are streams of electrons. (see Alpha Particle; Beta Particle).

The spectrum of particle and electromagnetic radiations ranges from the extremely short wavelengths of cosmic rays and electrons to radio waves hundreds of kilometers in length (see Electron). Between these limits the spectrum includes gamma rays and hard X rays ranging in length from 0.05 to 5.0 Å. Softer X rays merge into ultraviolet light as the wavelength increases to about 500 Å, and ultraviolet, in turn, merges into visible light, with a range of 4000 to 8000 Å (see Light). Infrared heat waves are next in the spectrum (see Heat) and merge into microwave radio frequencies between 1 million and 4 million Å. From the latter figure, which equals 0.4 mm (0.016 in), to about 15,000 m (about 49,200 ft), the spectrum consists of the various lengths of radio waves; beyond the radio range it extends into the low frequencies of wavelengths measured in ten thousands of kilometers.

Ionizing radiation has different penetrating properties that are important in the study and use of radioactive materials. Naturally occurring alpha rays are stopped by the thickness of a few sheets of paper or a rubber glove. Beta rays are stopped by a few centimeters of wood. Gamma rays and X rays, depending on their energies, require thick shielding of a heavy material such as iron, lead, or concrete. See Radioactivity.

See also Nuclear Energy; Particle Accelerators; Particle Detectors; Quantum Theory.