Red Hz Infrared Hz Oxygenated hemoglobin absorbs weakly in the red (hence its re

Question

Red Hz Infrared Hz Oxygenated hemoglobin absorbs weakly in the red (hence its redcolor) and strongly in the near infrared, whereas deoxygenatedhemoglobin has the opposite absorption. This fact is used in a pulse oximeter to measure oxygen saturation in arterial blood.The device clips onto the end of a person's finger and has twolight-emitting diodes --a red (670 nm) andan infrared (980 nm)-- and a photocellthat detects the amount of light transmitted through the finger ateach wavelength.(a) Determine the frequency of each of theselight sources. Red Hz Infrared Hz (b) If 64% of theenergy of the red source is absorbed in the blood, by what factordoes the amplitude of the electromagnetic wave change? [Hint: Theintensity of the wave is equal to the average power per unit areaas given by the equation given below.] 3

Explanation / Answer

(a)
   we know that
   v = f
   the frequencies of the two light sources are
   fred = c / red
         = (3 x108 m / s) / (670 x 10-9 m)
         = ....... Hz
   finfra red = c / infrared
         = (3 x108 m / s) / (980 x 10-9 m)
         = ....... Hz
(b)
   we know that the intensity of the electromagnetic waveis proportional to the square of

   its amplitude so if 64% of the intensity of the redlight is absorbed then the intensity of

   the emerging wave will be (100 - 64) % = 36 % ofthe incident intensity so we can

   write
   If = 0.36 Ii
   Emax,f / Emax,i =(If / Ii)   
                        = .........

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