Basic Principle
WARNING: This page is about a different car, the 2013 Kia Sportage and 2012 Kia Sportage. However, it is still accessible from the selected car via links, so may be relevant.
Emitted Beam from luminosity diode is reflected entirely against the windshield exterior, and then turn into photo diode.
If there is water on the windshield exterior, beam separates optically, and the degree of remained beam is measured in the photo diode.
When there is water in the windshield, it means beam is not reflected all, so the degree of lost beam indicates the degree of glass surface wet.
NOTE:
Rain sensor consist of two luminosity diode, two photo diode, optic fiber and coupling pad.