Question

A pulse oxymeter determines a person’s oxygen saturation, that is what fraction of the hemoglobin in their bloodstream carries an oxygen molecule. Hemoglobin molecules that are carrying oxygen strongly absorb 940 nm waves, while hemoglobin without oxygen instead strongly absorbs 660 nm light. So, pulse oxymeters contain two light sources at 660 nm and 940 nm. Typically the device is (painlessly!) clipped onto person’s finger, and by comparing how much 660 nm and 940 nm light is transmitted through the finger, it can determine the oxygen saturation.

a.) What type of electromagnetic waves are the 660 nm and 940 nm waves? Could you see either of them?

b.) It turns out that the signals seen by an oxymeter vary with time due to one’s heartbeat. So typically each light is quickly pulsed on and off 30 times per second and the oxymeter looks at the peak values observed. In a pulse of light that lasts 1/30th of a second, how many 660 nm wavelengths are within the pulse?

c.) What is the ratio of the speed of the 660 nm waves over the speed of the 940 nm waves? Explain.

Answer 1

a) While visible light is in the range of 400 nm to 700 nm, waves at 660 nm fall into the visible region of EM waves( and is seen as red light), while wave emited at 940 fall more on the Infra Red type of radiation.

b) wavelength = 660 nm

time (t) = 1/30 sec = 0.033 sec

wavelengths that can fit in this pulse,

660 / 0.033 = 20,000

ans : 20000 wavelengths.

c) Wavelength,

And v ( velocity) = distance / time.

thus v at 660 = 660 * 10^-9 / 0.033 = 20,000 * 10^-9

and v at 940 = 940 * 10^-9 / 0.033 = 28,484.84

Thus required ratio : 5 : 7