I can't get all scientific on you like some of the other members, but I just know that it's the type of light that is most preferred by the plant in her two different stages (vegging and flowering). Remember that you are trying to replicate the natural growth of a plant outdoors, except indoors.

As for you cup, make sure you either raise your soil level or trim the cup down so air can circulate better. With those high sides you're going to have a lot of stale air and possible mold growth.