Just beginning to rain, the wipers will get to work. Yes, not anyhow, but given the intensity of this rain itself. But who gives them a command instead of a driver?
All the sacrament takes place in a small box hidden behind a rearview mirror. Depending on the model, an infrared LED and one to three photodetectors are placed inside. Look at the diagram. The LED emits an invisible beam at an acute angle to the surface of the windshield. If the street is dry, all the light is reflected back and gets on the photodetector (the optical system is designed this way). Since the beam is modulated by pulses, the sensor will not react to extraneous light, like a TV that does not “see” someone else's remote control. But if there are drops or a layer of water on the glass, the conditions of refraction change and part of the light goes into space. This is detected by the sensor and the controller calculates the appropriate wiper mode. Along the way, a tricky device can close the sunroof in the roof and raise the windows.
The diagram shows two more photodetectors. Although they are integrated into a common housing with a rain sensor, they have nothing to do with it. The one that looks up is useful for automatically turning on the headlights when it is getting dark or the car enters a tunnel. The second, "forward looking", will save the driver from the need to switch from high beam to low beam. Whether these functions are involved depends, understandably, on a particular car model.