ABSTRACT
Electromagnetic energy transforms into some kinds of energy by the absorption process. Some optically active materials are necessary for the quantitative changes in energy. Optical absorption occurs by the resonance between the frequencies of the incident radiation and the oscillation in the material. The dielectric materials that fit for the evaporation technique became optical materials. The wavelength region treated in optics was expanded to both infrared and ultraviolet regions. Some kinds of metals such as tungsten and tantalum have been used in the vacuum ultraviolet region, and some kinds of ionic crystals such as NaCl and KCI, and some kinds of semiconductors like Si and Ge have been used in the near- and middle-infrared regions. Oxide silicate glasses are used widely for lens materials, because the refractive indices of the oxide silicate glasses can be varied almost continuously by changing the composition, and the transmittances of the oxide silicate glasses are very high compared with those of other materials.
