The third and most advanced form of fingerprint sensing uses ultrasonics. Much like the ultrasonic scanners used for medical purposes, an ultrasonic sound pulse is transmitted to your finger and the reflected pulse is measured.
Bats, whales and dolphins use ultrasonic to understand the shape of their surroundings; the sensors on a smartphone use it to understand the 3D shape of the ridges in your fingerprint. It can even work through the phone screen.
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