How do automatic windshield wipers know when to wipe?
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When a ray of light goes between glass and air, it will “refract”. Change the angle it travels at. You might see this effect making a straw in a glass of water appear bent.
If the angle is sharp enough, the refraction means the light gets reflected inside the glass. This is what happens normally. We have an infra red light, and a sensor that expects to detect the reflected light.
However, if there’s a drop of rain on the glass, we aren’t looking at light going from glass to air. It’s going from glass to water. This causes a much smaller amount of bending. The light doesn’t get reflected back. The sensor detects it’s not receiving light and tells the wiper to activate.
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