eli5 – How ”green light” from smartwatches/bands ”reads” your HR?

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the above

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Your blood is red. That means it absorbs every color but red, or the other way around, can only reflect red light.

Light is on a spectrum of frequencies. Green light has almost all its energy between 500 and 600 nm in wavelength. Imagine those photons being green balls.

Red light has a wavelength of 650 to 700 nm. Those are red balls.

Now your blood really likes to throw (reflect) red balls (photons) and likes to keep (absorb) green balls (photons). If you now have one blood cell and you throw a bunch of green balls at it, it will try to catch all of them and stash them. It can’t however catch all of them, so some hit the wall behind it and bounce back to you. If our bloodcell now gets all its friends and cousins, there are way more of them all trying to catch the green balls. Therefore, fewer green balls bounce back. So, if you always throw the same amount of balls, you can guess how many blood cells there are by counting the balls that come back.

In technical terms, the more purely red stuff (=blood) there is, the fewer green photons can be reflected (the tissue of your body reflects way more green light), so the intensity of the green light decreases. If you now measure this intensity, you get a curve that is inversly proportional to the amount of blood in the tissue, which is proportional to the pressure and therefore the heart beat.