Word: afterglow
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...makes them glow. Some materials are affected by sound, radio waves, slow oxidation, cathode rays, or shaking. But all commercial luminescent pigments are photoluminescent: they glow only after stimulation by light. If the pigment glows for several minutes or hours after exposure to light, it is phosphorescent. If the afterglow is very brief-perhaps only 1/10,000th of a second - the pigment is fluorescent. Hence fluorescent substances glow only when continuously exposed to invisible ultraviolet rays ("black light"), which they reflect as visible light...
...both types of photoluminescence are similar physical phenomena. Many luminescent materials are crystalline. Impact of light waves moves some electrons away from their normal position within the latticed clumps of molecules called phosphors. Then, as each electron moves back to its original orbit, it emits light waves. Length of afterglow depends upon the time taken by the electrons in returning...
...their pure form some luminescent pigments glow quite feebly, but they brighten up startlingly when only .0006% of certain metals such as copper are added. Yet if a similar infinitesimal .0006% of iron is also present, the afterglow is dimmed by one-third. Since iron could easily be floating about the laboratories as dust, great care must be taken to guard the purity of the ingredients while they are being compounded in electric furnaces. Result: luminescent pigments are costly-price last week was from $12 to $25 per lb., or $10 to $60 per gallon for paints...
...Last week Drs. Joseph Kaplan and S. M. Ruben of U. C. L. A. told how they brought the auroral colors down to earth. They put gas molecules in a tube, stirred them up with a high-frequency discharge, then snapped off the current with extreme suddenness. The brief afterglow of the gases they caught in a spectroscope. They found the colors to be not only those of excited oxygen and nitrogen, the most plentiful components of the air, but also of helium, which makes up only .0005% of the atmosphere and is exceeded in scarcity only by xenon. Conclusion...
...with great emotion that I read your letter in TIME. The sentiments expressed therein go straight to my heart. I am a Frenchwoman who in the afterglow of 1918 married one of those soldiers who came to help us at that time...