You know those things that they clip to your finger when you’re in the hospital that are connected to a cable that connects to a machine that beeps. Well they’re called pulse oximeters and they measure the amount of oxygen in your blood and your heart rate.
What’s really interesting is how they do this: one side has a pair of LEDs, each of a different wavelength, that shine light through your finger; on the other side is a detector that detects this light. Because oxygenated blood and deoxygenated blood absorb the light at different rates, the changes in the reception of the light can be used to calculate both oxygen saturation in the blood and heart rate. All without needing to stick anything inside you or draw any blood. Neato.