How rain-sensing windscreen wipers work

If you’re lucky enough to be shopping for a new car, the salesperson will be sure to point out the marvellous rain-sensing wipers. So try an experiment: Ask him or her how these marvels work.

You’ll probably be told they work via a sensor. Tremendous; that’s like telling you that your refrigerator works by keeping stuff cold.

So here’s how rain-sensing wipers REALLY work: The dingbat salesperson is right inasmuch as there’s a sensor involved. It’s usually mounted up near the interior mirror (where it doesn’t get in your sightline) and is aimed at the windscreen. This sensor sends out an infra-red beam which, when it hits the dry windscreen, is reflected back almost completely.

But as rain-drops fall on the windscreen, they act as prisms, scattering the light so that only some of the infra-red gets back to the sensor. And when that happens, the car’s on-board computer switches on the wipers. As the wipers clear the screen, infra-red reflection is restored and the wipers shut down until the next drops fall on the windscreen. Beautiful.