Instead of varying the speed of the wiper with a few limited gears, the rain sensor adjusts the speed of the wiper stepless. It has a light-emitting diode called an LED that sends far infrared light.
When the surface of the glass is dry, the light is reflected back almost 100%, so the diode can pick up a lot of the reflected light. The more rain falls on the glass, the less light is reflected back,
with the result that the wipers move faster.