The range of so-called radiant energy is very broad, extending from radiations having waves which are miles in length, on through radio frequencies, infra-red (popularly called heat), visible, ultraviolet, roentgen and gamma rays until the cosmic rays recently discovered by Millikan are reached. The wave lengths of radiation, other than in the case of electric waves, are generally recorded either in terms of the angstrom unit or in millimicrons. An angstrom unit is one ten-millionth part of a millimeter. The millimicron, mμ, is ten times larger than the angstrom unit, A., and the micron, μ, is 10,000 times greater. In table 1 are given the approximate limits of various spectral regions.
The various wave lengths of radiant energy are due to vibrations or oscillations of matter in the molar, molecular, atomic or electronic states. Electric waves are due to molar vibrations, infra-red radiations to disturbances in molecules and atoms, and
SHEARD C. ULTRAVIOLET RADIATION AND ITS TRANSMISSION BY VARIOUS SUBSTANCES. JAMA. 1927;88(17):1315–1318. doi:10.1001/jama.1927.02680430017007