Question

1. Lasers made with NdYAG (Neodymium-doped Yttrium Aluminum Garnet) crystals, which emit a beam with a wavelength of 1064 nm, have many applications, including their use to remove skin cancers.

a) What type of electromagnetic waves are these (visible light, x-rays, ultraviolet radiation, etc.)? Can you see a 1064 nm beam with your eyes? Are there any safety precautions that might be especially important with a laser like this?

b) What is the frequency and period of this light?

2. A pulse oxymeter (see image) 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

1) a) A wavelength of 1064nm lies in the Infrared spectrum of electromagnetic radiation. The visible range of electromagnetic radiation for humans is from 400nm to 700nm and so an Infrared photon of 1064nm is not visible.

Safety precaution for using such lasers for experiment is to first align the Optics using a laser in visible spectrum (green preferably). Once the beam paths are as desired, replace the green laser with the Infrared one so that major alignment with it is avoided.

Use laser viewing cards for infrared spectrum.

b) Frequency is related to wavelength by:

For 1064nm,

Period for this wave is: T = 1/f = 3.5466 x 10^-15 s.