It's Monday! Thought I'd start the week off with a boring physics lesson. Why not? Physics is an integral part of photography's art.
Light is a naturally occurring, transverse, electromagnetic wave that can be seen by humans and other species.
Light is produced two ways: 1) Incandescence, i.e., the emission of light from "hot" matter and 2) Luminescence, that is, the emission of light when excited electrons fall to lower energy levels. (Luminescent light occurs in matter that may or may not be "hot.")
(Caution: NSFW pic)