system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why do fitness trackers and smart watches need the green lights/lasers to operate, and why are they always green?
gpt
They seem to be using a technology called Photoplethysmography (PPG) where green (and sometimes infrared) light is flashed onto the skin and sensors measure the return amount.

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Blood is red (duh) and it absorbs green light. During a heartbeat, there is more blood flow, than in between heartbeats. 

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More green light returned (no heartbeat)  
Less green light returned (heart just beat)  


Do that over a few seconds and you get a pulsrate.  
And the light used is green, because it seems to give the best (most accurate) results compared to red or blue

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