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If you own a smartwatch, odds are you have removed the device from your wrist and noticed the underside flashed green for a second or two. It doesn't matter if it was an Apple Watch Ultra 3 or a budget alternative device like the Amazfit Active Max; they all use green lights because it makes it easier to detect your pulse.

Smartwatches measure your pulse using a process known as photoplethysmography. This technique detects how much blood volume changes in tissue beds, specifically the skin. Capillaries expand and contract while transferring blood to your organs, including your skin, and these movements coincide with your heartbeat. Smartwatches shine a light onto the skin, record how much light returns and how quickly the amount changes, and crunches numbers to give your pulse. But why do smart watches use green light? Because blood is red. No, seriously.

We all know blood contains hemoglobin — iron-carrying molecules that turn red when exposed to oxygen. But why do we see hemoglobin as red? Because it absorbs most light and only reflects light in a specific wavelength: red. Red objects absorb green light like crazy, so when capillaries are full of blood, they gobble up green light. This makes it easier for smartwatches to differentiate between capillaries swollen with blood and capillaries that aren't. Furthermore, green light doesn't penetrate your skin as deeply as other colors. On the surface, that sounds like a disadvantage, but that's exactly the point: Keeping measurements to the surface of the body lets smartwatches easily parse data and provide more accurate measurements.

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Depending on your smartwatch model and its functions, you might notice certain other colors of light on the underside. Devices such as Apple Watches can glow red, and the reason is more or less the same as why they glow green. Well, kinda.

In order for smartwatches to measure your blood oxygen, they use a process known as pulse oximetry, which shines a red light and infrared light onto your skin. However, unlike the device's photoplethysmography function and its green light, pulse oximetry isn't concerned with how quickly the amounts of returned light change but just how much returns in general. This is why red and infrared lights are so important.

As previously stated, hemoglobin consumes green light wavelengths and reflects red light wavelengths. However, the strength of a hemoglobin's red hue, and how it interacts with different light wavelengths, depends on how much oxygen it contains. Oxygenated hemoglobins have a strong red color that absorbs more infrared light and less red light, whereas deoxygenated hemoglobins have the opposite behavior. Smart watches calculate how much red and infrared light is absorbed versus how much isn't and uses that data to calculate your blood oxygen levels. If you have an Apple Watch that doesn't include a blood oxygen monitoring app, that's probably because the device only has green lights capable of photoplethysmography, not the red and infrared ones needed for pulse oximetry.