The published values of the spectral diffuse reflectance of magnesium oxide indicate that it should reflect more highly in the red than in the blue. Using a modification of the method described by Preston, we have measured the absolute value of this quantity between λ=0.24μ and λ=0.85μ. A very carefully prepared surface of MgO on silver has a reflectance of about 0.98 at λ=0.54μ, where there is a very flat maximum. The reflectance of a fresh surface falls to less than 0.96 at 0.4μ, but exposure to strong ultraviolet light bleaches it, the final result being a reflectance increasing continuously from the near-infrared to about 0.4μ, where it begins to fall sharply, reaching 0.96 at about 0.25μ. A sphere lined with such a bleached surface raises the color-temperature of Planckian radiation a little, and the observed color agrees well with that predicted from the new results. It is believed that these results are reliable to ±0.002 from λ=0.4μ to λ=0.7μ, but different surfaces prepared with equal care may differ in reflectance by at least 0.005.
The bleaching is presumed to be caused by the decomposition of small amounts of magnesium nitride.
© 1951 Optical Society of AmericaFull Article | PDF Article
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