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 (653 nm) and an infrared (935 nm)—and a photocell that detects the amount of light transmitted through the finger at each wavelength.

If 64% of the energy of the red source is absorbed in the blood, by what factor does the amplitude of the electromagnetic wave change?