What is the effect of opening airplane bay doors on the radar cross section of the airplane?
Radar waves are sent back to the radar by two mechanisms, reflection and diffraction.  Opening a bay door may present a surface that is angled such that waves will be reflected to the radar.  As for diffraction, low observable (stealth) aircraft are designed so that a radar wave encountering the aircraft's skin will travel along the skin and be gradually absorbed.  Opening the bay doors creates a break in the skin, causing waves on the skin to diffract.  Some of the radar energy is returned to the sender.

For an amateur looking at "stealth" diffraction is often not accounted for.