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The laws of physics govern the energies used by therapeutic modalities. This appendix presents an overview of these physical properties. Modality-specific physical properties are discussed in the relevant chapters of this text.

The Electromagnetic Spectrum

Various forms of energy are constantly bombarding us: the light from the sun, the heat from a fire, and the waves emitted from radio transmitters. This energy, known as electromagnetic radiation, is produced by virtually every element in the universe and is characterized by the following traits:

  • Transports energy through space

  • Requires no transmission medium

  • Travels through a vacuum at a constant rate of 300 million meters per second

  • Does not have mass and is composed of pure energy

Each form of energy is ordered on the electromagnetic spectrum on the basis of its wavelength or frequency (Fig. A-1).

Figure A-1.

A Graphical Representation of the Electromagnetic Spectrum. (Adapted from Illuminating Engineering Society of North America Lighting Handbook, New York, ed 8, 1993.)

Regions of the Electromagnetic Spectrum

The energy's wavelength uniquely defines each portion of the electromagnetic spectrum. The reference measure for wavelength is the meter (Table A-1).

TABLE A-1Units of Measure for Wavelengths Relative to the Meter (39.37 in.)

Ionizing Range

Energy within the ionizing range of the electromagnetic spectrum is characterized by the relative ease with which atoms can release free electrons, protons, or neutrons. Ionizing radiation can easily penetrate the tissues and deposit its energy within the cells. If this energy is sufficiently high, the cell loses its ability to divide, eventually killing the cell.

Ionizing radiation is used diagnostically in obtaining radiographs (below the threshold required for cell death) and therapeutically in radiation treatment for some forms of cancer (above the threshold). Because ionizing radiation is hazardous, the total dose of exposure must be tightly monitored and controlled. Energy found in this portion of the electromagnetic spectrum is used only under closely controlled circumstances.

The Light Spectrum

This portion of the spectrum encompasses ultraviolet, visible, and infrared light energy. Electromagnetic radiation possessing a wavelength between 380 and 780 nm forms the spectrum of visible "white" light. White light is the combination of seven colors, each representing a different wavelength on the spectrum. These seven colors, ranked from the shortest to the longest wavelength, are ...

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