blood is red because it reflects red light and absorbs green light, they developed a system to monitor the amount of blood flowing through a wrist’s arteries by flashing green LED lights hundreds of times per second. Each heartbeat increases the blood flow through the arteries. As the blood moves, it absorbs more green light. Between heartbeats, the absorption of green light decreases. Those differences could be calculated in real time by the sensors and algorithms to determine the number of heartbeats per minute. The team developed a separate infrared sensor that shined a light onto the wrist
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