Oxygenated hemoglobin absorbs weakly in the red (hence its red color) and strong
ID: 1332405 • Letter: O
Question
Oxygenated hemoglobin absorbs weakly in the red (hence its red color) and strongly in the near infrared, whereas deoxygenated hemoglobin 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 two light-emitting diodes --a red (665 nm) and an infrared (935 nm)-- and a photocell that detects the amount of light transmitted through the finger at each wavelength.
(a) Determine the frequency of each of these light sources.
(b) If 65% of the energy of the red source is absorbed in the blood, by what factor does the amplitude of the electromagnetic wave change? [Hint: The intensity of the wave is equal to the average power per unit area as given by the equation given below.]
Explanation / Answer
a)
frequency = c/wavelength
c = speed of light wave = 3*10^8
(a)
fred = (3*10^8)/(665*10^-9) = 4.51*10^14 Hz
finfrared = (3*10^8)/(935*10^-9) = 3.2*10^14 Hz
(b)
Intensity of refleted wave = I2 = I1-0.65I = 0.35I1
I1 / I2 = E2^2 - E1^2/(E1^2)
(E2/E1 = (sqrtI2 /sqrtI1
(E2/E1 = sqrt0.35
E2/E1 = 3/5
E2 = (3/5)*E1 <<--answer
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