The dictionary definiton of light is:
the natural agent that stimulates sight and makes things visible.
I'll paraphrase/give a scientific definition:
Light is made of subatomic particles called photons, which jumps electron shells when they're excited, effecting the electromagnetic spectrum, creating visible light.
Hope this helps. Might not as I'm not perfect on this topic yet.
Definitely definite mass because liquids can take on any shape and can have different volumes.
I think it might just might be e
The transitions which fall to the lowest principle position release the greatest energies. In this case, this would be the transition from the 5p to the 3s orbital (a Paschen transition).
Hope this helps!