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Quite technically it is condensation that causes the water on your glass, evaporation is the removal of the water all together. As long as it is a full glass cover, you can rest the lamp fixture on the glass (I am assuming you are talking about flourescent or other cool light source)
Normal output flourescents aren't that hot, but I have 4 VHO (very high output) flourescents on my tank, and they are HOT. I wouldn't set those on glass.