system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 How can a blood oximeter read your level of oxygen in the blood without requesting a drop of blood?

My mom bought one of those recently and I still can not figure out how that thing works without a drop of blood.

Edit: Oh this really did blow up. Thank you all for responding. I have more knowledge that I anticipated on this little thing now. Very useful.
gpt
The trick is they shine two colors of infared light in your skin, and then look at the reflection.

The two colors reflect very differently off oxygenated and unoxygenated blood, and the ratio between the brightness can be used to figure out the oxygen concentration in your blood.