Thin films of alkali metals opaque to visible light are highly transparent in the ultraviolet region. The point in the spectrum at which transparency commences moves toward shorter wave-lengths with decreasing atomic number as follows: Cs 4400; Rb 3600; K 3150; Na 2100; Li 2050A. The transparency continues as far down the spectrum as the investigations have gone (1860A). In this region the phenomenon of plane polarization by reflection is observed. The reflecting powers of these metals for different wave-lengths have been measured. From these measurements and also from the Brewsterian angles for plane polarization, the refractive index of a potassium film has been calculated. The values range from 0.90 at 2147A to 0.50 at 3100A. Since the refractive index is less than unity total reflection takes place, although the critical angle is not sharply defined. Interference maxima and minima in the spectrum of light reflected from a metal film permit a rough determination of the film thickness.

• Received 23 June 1933

DOI:https://doi.org/10.1103/PhysRev.44.353