The following photo depicts an infrared proximity sensor:

It consists of two major parts—the sensor and the transmitter. The transmitter emits IR waves; these Infrared (IR) waves then hit the object and come back to the sensor, as depicted in the following diagram.. 

Now, as you can see in the preceding diagram, the emitted IR waves bounces back from a surface at a different distance from the sensor, then they makes an angular approach to the sensor. Now, because the distance between the transmitter and the sensor is fixed at all points of time, the angle corresponding to reflected IR waves would be proportional to the distance it has traveled before bouncing off. There are ultraprecise sensors in the IR proximity sensors that are capable of sensing the angle at which the IR waves approach it. By this angle, it gives the user a value of distance corresponding to it. This method of finding distance is named triangulation, and it has been used widely in the industry...