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 (638 nm) and an infrared (974 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.