Index of Refraction of Magnesium Oxide

In 1935, shortly after moderate sized artificially grown crystals of magnesium oxide became available, two papers were published dealing with the optical properties of this material. 2 Strong and Brice measured the index of refraction of two prisms, cut from the same crystal, by photographic and visual means over the range from 2536 to 7065 A. Barnes, Brattain, and Seitz investigated the transmittance and reflectance of this material in the infrared region from approximately 1 to approximately 30 /x. They published a curve showing the index of refraction as calculated by means of the Fresnel reflection equations from the measured reflectance. Some time ago two small prisms of magnesium oxide crystals of excellent optical quality were sent to the National Bureau of Standards by Elias Burstein of the Naval Research Laboratory for measurements of the index of refraction and the thermal coefficients of the index of refraction. These prisms, although rather small, were of high optical quality and entirely adequate for precision measurements of index in the visible region of the spectrum. Because of the high sensitivity of the lead sulfide photoconducting cell in the region from about 0.3 to about 2.6 /z, it was found practicable to measure the index of refraction for several ultraviolet and infrared lines of the mercury and cadmium spectra with good accuracy. Beyond 2.6 ju it was possible, by using wide slits and a Golay pneumatic detector, to get data of fair accuracy at the wavelengths of several absorption bands of polystyrene and carbon dioxide. Due to the limitations of the apparatus, the range of the measurements was from 0.36117 \x (cadmium) to 5.35 fi (polystyrene). According to the results referred to in footnotes 1 and 2, magnesium oxide is usefully transparent from 0.2200 to 9 /x.