Rain-sensing wipers may seem like arcane magic, but in principle they’re pretty simple.
On most cars the system consists of a bank of infra-red LEDs and a bank of light-measuring devices, mounted near the interior mirror.
The little infra-red LEDs shine constantly, bouncing beams through the glass. The angle the lights are set at brings into play a physics principle called ‘total internal reflection’. Basically, the beam of infrared light bounces off the top surface of the glass and back down to the sensor. When the windscreen is dry, most light is returned back to the sensor.
When water droplets fall on the glass they act like lenses, causing the light that would have been reflected straight back to the sensor when it was dry to be bounced around inside the droplet instead. This causes much less light to be returned to the sensors. The car’s computer interprets this sudden drop in light return as rain and switches on the wipers. The more water on the screen, the less light returned, which triggers faster wiping.