Ultraviolet divergence short distances. The name comes from the earliest example of such a divergence, the "ultraviolet catastrophe" first encountered in understanding blackbody radiation. According to classical physics at the end of the nineteenth century, the quantity of radiation in the form of light released at any specific wavelength should increase with decreasing wavelength—in particular, there should be considerably more ultraviolet light released from a blackbody radiator than infrared light. Measurements showed the opposite, with maximal energy released at intermediate wavelengths, suggesting a failure of classical mechanics. This problem eventually led to the development of quantum mechanics. The successful resolution of the
Umkehr effect Umkehr effect The Umkehr is the time variation of the ratio of the scattered intensity at two different wavelengths. The word means 'reversal' in German. The Umkehr effect is observed when measurements are made with ultraviolet spectrophotometer of the ratio of the zenith sky light intensities of two wavelengths in the solar ultraviolet when the sun is near the horizon. The shorter of two wavelengths (intensity "I") is strongly absorbed and other (intensity "I' ") is weakly absorbed. If the value of log("I"/"I' ") is plotted against the sun's zenith angle, it is observed that this log-intensity ratio decreases as