For some reason, something just came to my mind about a strange result I got when doing a photoelectric effect experiment early this spring. I had "monochromatic" LEDs (I am most suspicious about them) shining into a hole in a "box" that contained the photo sensitive "vacuum tube thing" (the thing that contains the photo emissive electrode in glass). If I shined the LED directly into the hole (i.e. the plane of the junction in the LED ~parallel to the plane of the hole), then I get the photo current to vanish at some voltage repeatably. But, I found that, keeping that voltage, if I tilt the incidence to some value ~20o off or so, then the the photo current would spike way up, and it would not respond anywhere at all as sensitively to a change in voltage. When I did set the voltage to eliminate the resulting photo current, and then left the tilt, the setting was repeatable. This occured for all four of the colours (visibly red, blue, green, and yellow) and one UV LED. The voltage settings were repeatable after dismantling the setup and rerunning the experiment the next day. So, after that long-winded exposition, does anyone have any suggestions for an explanation? I suspect the diodes, but the "box" has a lot of control circuitry that makes me suspicious as well. I may be able to dig up the data (but I doubt it).