An IR Autofocus System Has Emitter and Detector Apertures Separated $\text { (x.x m) }$

Essay

An IR autofocus system has emitter and detector apertures separated by 1cm and modeled as pin holes.The light reflected from an object at range r casts a spot that is detected at a displacement of 0.1mm from the detector axis.The detector lies 0.5cm behind the pin hole.The value of r equals _____ $\text { (x.x m) }$