Alternating Current Circuits and Electromagnetic Waves223(b) The intensity of an electromagnetic wave is proportional to the square of its amplitude. If 67% of the incident intensity of the red light is absorbed, then the intensity of the emerging wave is ()100%67%33%−=of the incident intensity, or 0.33fiI=I. Hence, we must have max,max,0.33ffiiEIEI==0.57=21.46If Iis the incident intensity of a light beam, and Iis the intensity of the beam after passing through length Lof a fluid having concentration C of absorbing molecules, the Beer-Lambert law states that 0()100logI ICLε= −where εis a constant. For 660-nm light, the absorbing molecules are oxygenated hemoglobin. Thus, if 33% of this wavelength light is transmitted through blood, the concentration of oxygenated hemoglobin in the blood is ()102log0.33HBOCLε−=( The absorbing molecules for 940-nm light are deoxygenated hemoglobin, so if 76% of
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