Which technology do rain sensors use to detect moisture?

• A. Infrared beams
• B. Ultrasonic waves
• C. Capacitance changes
• D. Mechanical contacts

Answer: (A)

Rain sensors detect moisture using optical sensing with infrared light. An infrared emitter sends a beam across the windshield to a receiver. When the windshield is clear, most of the infrared light reaches the detector; when droplets are present, they scatter and absorb the light, causing the signal at the receiver to drop. The control unit interprets that change as rain and can trigger or adjust the wipers. The other options aren’t typical for detecting windshield moisture: ultrasonic would use sound waves and echoes, which isn’t how rain on glass is detected; capacitance changes would require a specific sensor layout to measure dielectric changes from moisture, which isn’t the common automotive rain-sensing method; and mechanical contacts would need water to bridge a switch, which is unreliable and prone to wear.