Light and the Electromagnetic Spectrum

The color of minerals is one of their most fascinating properties and is frequently the first thing that draws attention to the mineral. Color can originate in a number of ways from the presence of a cation or anion that is colored, from the presence of an impurity that is colored, or from a defect in the arrangement of the ions in the solid. The color is always a result of the transition of electtons in the ions or the crystal lattice from one energy level to another. Before we explain this movement of electrons we need to review the nature of light and the electromagnetic spectrum. 

The wavelength is the distance from crest to crest or from trough to trough. 

Ionic Compounds, By Claude H. Yoder Copyright 2006 John Wiley Sons, Inc. 

The wavelenglh and the frequency are inversely related that is, as the frequency increases, the wavelength decreases, and this relationship is given by the equation 

Another important relationship for light was discovered by the German physicist Max Planck. The energy of light is proportional to its frequency, with the proportionality constant, the Planck constant h, having a value of 6.6 x 10 J-s  

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In the electromagnetic spectrum, the ultra violet region is between that of X-rays and visible light. This corresponds to the energies hv ot one hundred to a few tens of electron-volts (wavelengths from 180 to 400 nm). 

Spectroscopy, or the study of the interaction of light with matter, has become one of the major tools of the natural and physical sciences during this century. As the wavelength of the radiation is varied across the electromagnetic spectrum, characteristic properties of atoms, molecules, liquids and solids are probed. In the... 

The so-called peak power delivered by a pulsed laser is often far greater than that for a continuous one. Whereas many substances absorb radiation in the ultraviolet and infrared regions of the electromagnetic spectrum, relatively few substances are colored. Therefore, a laser that emits only visible light will not be as generally useful as one that emits in the ultraviolet or infrared ends of the spectrum. Further, witli a visible-band laser, colored substances absorb more or less energy depending on the color. Thus two identical polymer samples, one dyed red and one blue, would desorb and ionize with very different efficiencies. 

Neodymium and YAG Lasers. The principle of neodymium and YAG lasers is very similar to that of the ruby laser. Neodymium ions (Nd +) are used in place of Cr + and are often distributed in glass rather than in alumina. The light from the neodymium laser has a wavelength of 1060 nm (1.06 xm) it emits in the infrared region of the electromagnetic spectrum. Yttrium (Y) ions in alumina (A) compose a form of the naturally occurring garnet (G), hence the name, YAG laser. Like the ruby laser, the Nd and YAG lasers operate from three- and four-level excited-state processes. 

The scattering of visible light by polymer solutions is our primary interest in this chapter. However, since is a function of the ratio R/X, as we saw in the last section, the phenomena we discuss are applicable to the entire range of the electromagnetic spectrum. Accordingly, a general review of the properties of this radiation and its interactions with matter is worthwhile before a specific consideration of scattering. 

In 1957, this team of brothers-in-law started working together on Townes s idea for an optical maser. They found atoms that they felt had the most potential, based on transitional probabilities and lifetimes. However, there was still one major problem In the visible light portion of the electromagnetic spectrum, atoms don t remain in an excited state as long as... 

Absorption spectrum (Section 12.5) A plot of wavelength of incident light versus amount of light absorbed. Organic molecules show absorption spectra in both the infrared and the ultraviolet regions of the electromagnetic spectrum. 

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