Infrared Detectors!!

Hey,I am pretty new to electronics.
I plan to make a line follower with obstacle avoidance for which I would need an infrared emitter detector..
This is what I already know..The ir emits rays that strike an obsatcle and the reflected rays fall upon the detector which is a photodiode.considering the fact that a photodiode works on reverse bias where breakdown occurs and the reverse current is generated.(electric signal)
Now what I want to know is what happens to this reflected ray..???What does a photodiode do to it???
Does it have any specific voltage like (say 1.7volts or something)??
PLZ REPLY!!!