Optoreflective sensors like this one are used in a myriad of applications, from presence detectors for small objects, to mobile robots like Sumo line trackers or robots.

 The QRD1114 is an optoreflective sensor consisting of an infrared emitting diode and an NPN type phototransistor mounted side by side in a plastic package.
 The phototransistor responds to the light emitted by the Infrared LED only when they are both on a reflective surface within the range of view of the phototransistor.