Reflection and Transmission of Ultra-Violet Light by Sodium and Potassium

Abstract

Reflecting power of K and Na, for wavelengths from 0.25 to 0.55 μ. (1) Normal incidence (10°). Opaque mirrors in contact with fused quartz were tested with monochromatic light from a mercury arc. A sodium photoelectric cell was used to compare the intensities of incident and reflected beams. It was found that sodium maintains a remarkably high reflecting power throughout the ultra-violet, increasing slightly from a flat minimum near 3600A to 80 per cent. at 2536A. The reflecting power of potassium on the contrary decreases rapidly with the wavelength, reaching the very low value of 11 per cent. at 2536A. A thin film of potassium deposited and kept at -190°C. gave a curve of the same form. Among the metals so far investigated in the ultra-violet, therefore, sodium and potassium have respectively the highest and lowest reflecting powers. (2) Polarized light at 45° incidence. The ratio of the reflecting powers for light polarized with the electric vector respectively parallel and perpendicular to the plane of incidence, was found for both sodium and potassium to have a maximum value near 3650A. There is also a common minimum at about 3341A which is far more pronounced in the case of sodium.