John ! If the Fiber you were playing with at the time....didn't Break !!! (Internally) then the bend you had given it probably was “beyond “the maximum bend Radius!!! for maximum light propagation That "Angle" is critical…. and the light (Maybe?) Was trapped between the Core and the Cladding ! bouncing around helplessly !!! and faintly visible at the other end ? or not at all !
Unless you guys are using top of the line fibers....(Glass Type) not the plastics cheap stuff; They will always suffer, some form of Light loss ! extreme Angle (twist and bends).... and it all depends on how you Feed the light into the fiber.... the Focal point at the light source could also influence on HOW good or HOW efficiently the light is travelling into your Fiber.... But I am talking DATA stream here... Visible light shouldn't be a big problem.... the Human eye can't tell ! Beside being ON or OFF !!!
Might as well pass many.... and hope they all pass light in the end !!!
behing a Physics teacher.... (IIRC) you know all about that stuff !