. Biological effects of radiation; mechanism and measurement of radiation, applications in biology, photochemical reactions, effects of radiant energy on organisms and organic products. Radiation; Biology. 258 BIOLOGICAL EFFECTS OF RADIATION 1,650 which absorbs all the incident radiation and reflects none. It gives, accordingly, the maximum radiation when heated. A heated enclosure such as a furnace with a small opening furnishes a practical means of obtaining this black-body radiation. Further details concerning black- body radiation are given on page 135. It is evident from Fig. 4 that a hea
. Biological effects of radiation; mechanism and measurement of radiation, applications in biology, photochemical reactions, effects of radiant energy on organisms and organic products. Radiation; Biology. 258 BIOLOGICAL EFFECTS OF RADIATION 1,650 which absorbs all the incident radiation and reflects none. It gives, accordingly, the maximum radiation when heated. A heated enclosure such as a furnace with a small opening furnishes a practical means of obtaining this black-body radiation. Further details concerning black- body radiation are given on page 135. It is evident from Fig. 4 that a heated solid, a tungsten filament, for exam- ple, offers a good source of light in the visible region of the spectrum but not in the ultra-violet. When atoms or molecules in the gas phase are subjected to extra en- ergy, the radiation may be emitted at specific wave-lengths, as distinguished from the continuous radiation of heated solids. The passage of an electrical current between metal elec- trodes causes the emission of a dis- continuous spectrum as in the case of the well-known mercury arc or iron arc. TYPES OF SPECTRA Spectra are classed as. 2 5 Wave- emission spectra and absorption spectra. Absorption spectra are obtained when white light of continuously varying wave-lengths is passed through ab- sorbing material. Light of certain wave-lengths passes through un- affected, while light of other wave- lengths is able to displace electrons (or atoms) in the molecules of the absorb- ing material. When the light rays Fig. 4- 5 ength (m^) 4.—Distribution of radiant energy are spread out in a spectrum with a emitted by a heated solid. prism or grating, those wave-lengths which have been absorbed are missing and a dark region results. The darkest regions in the absorption spectrum correspond to the greatest absorption and the most probable displacements within the molecule. As a rule, the absorption spectra, particularly in the case of solutions, are not so definite and the lines
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