Adafruit STEMMA Reflective Photo Interrupt Sensor - TCRT1000

An optical reflective sensor is a composite electronic device with two elements - an IR LED and an IR photo-transistor. The IR LED blasts light, and when something bounces the light back to the photo-transistor, the transistor turns on and the amount of current flowing through it increases. This makes the sensor great at detecting when something is in front of the sensor: when nothing is there the IR light never gets reflected back and the transistor stays off.