Literally the opposite of "in the dark" is "in the light":
I moved out of the darkness and into the light
However, it isn't natural to say "I'm in the light" in the same way we say "I'm in the dark".
Darkness is an absence of light. We tend more often to refer to the kind of light we are in, for example:
Don't run around in the dark - you should ...
You could easily use gather. Accumulate is a little bit more formal, but would also get the point across.
You might also want to think about using the verb to pool as in Pooled rain water will easily become a breeding ground for mosquitoes.
Sure you can call that swinging.
make regular movements forwards and backwards or from one side to another while hanging from a particular point.
That would seem to fit here and its certainly not the case that the object must be suspended from above. Doors can swing open, for example.
If you want to tell your child to stop swinging on the bed, normal ...
I would call that entire device "a power strip with a long cord."
If the fact that it has a long cord isn't important, I would just call it "a power strip." On the other hand, I might call the whole device "an extension cord," since it serves the purpose of an extension cord.
I would call the three parts of that device "the plug" (the part that plugs into ...