at what conditions will light, and an electron behave as a wave and as a particle?
You can never, under any conditions observe both wave-like and particle-like behavior at the same time. If you fire a single photon at a detector screen, you will see a single "dot", much as you would expect from a particle. Do that in a Young experiment many times, and you will find that the dots form a diffraction pattern, which is a wave-like property. Having said that, it is not observing a photon or electron acting both as a wave and particle at the same time. The duality is separated in that sense as far as observation goes.
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