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Here’s how it works—under normal conditions, the infrared light emitted by the LEDs reflects off the glass surface and travels back down to the photodiodes. This creates a steady light signal that the system can monitor. However, when rainwater begins to collect on the glass, an interesting phenomenon occurs due to light refraction.
Refraction and Total Internal Reflection
Refraction is the bending of light as it passes from one medium to another—such as from air to water. This change in direction and speed of light alters how much light is reflected, effectively breaking the internal reflection that is typically observed when light hits smooth surfaces.