Reflectance sensor circuit
That part is a phototransistor. It is one of the types of photodetectors. A phototransistor is similar to the normal BJT but the base region is exposed to the light and when it is exposed, the current will start to flow from Collector to Emitter. Instead of sending current to the base, the base is exposed to the light and when light strikes it activates the transistor.

For more info:- Phototransistors