being an ex-engineer I have always had a natural curiosity about how things work. It has cost me some embarrassment and trouble over the years when I have been unable to re-assemble something I took apart in my quest for knowledge.
So yesterday I was driving the car and it started to spit with rain - causing the wipers to start and then stop when the screen was dry. Having nothing better to do I started speculating how the rain is noticed by the car.
Rather than dismantle the car - I Googled when I got home.
A beam of infra red light from within the car is angled at 45o to the screen. Some of it is reflected back to the sensor. If the screen is wet, the light is deflected and it reduces the amount received by the sensor - at a certain level it causes the wipers to start up and remain working until the screen is dry.
So now we know.