Absorption of sunlight by atmospheric sodium
Scrimger, Joseph Arnold
A photometer has been built which is sensitive only to light in a very narrow spectral region near the sodium D-line8. The light is scattered by sodium vapour at low density and the scattered light recorded by a photoelectric spectrometer. The latter resolves the two lines and allows the effect of stray light to be eliminated. With sunlight the radiation detected is at the bottom of the Fraunhofer lines and the residual intensity there may be measured. It is found that this intensity decreases slightly (relative to the continuum) as the sun approaches the horizon. This effect is attributed to atmospheric sodium; the amount found by this method agrees well with that deduced from twilight measurements made the same day.