Oxygenated hemoglobin absorbs weakly in the red (hence its red color) and strong

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 (633 nm) and an infrared (900 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 68% 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 byI =

=

Bmax2.

Explanation / Answer

a) Frequency = speed of light / wavelength = 3x10^8 m/s / 633x10^-9m = 4.739*10^14 Hz
Frequnecy of infra red = 3x10^8/900*10^-9 = 3.33*10^14 Hz

b) The energy transported by a wave is directly proportional to the square of the amplitude of the wave.

If 68% (0.68) is absorbed, the remainder is 0.32

so the amplitude = sqrt 0.32 = 0.56

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