Every object* that is above 0 Kelvin (read: everything in the universe) radiates somewhere on the electromagnetic spectrum. Stick with me, and I'll explain why. But first, every object* also absorbs light somewhere along the electromagnetic spectrum. For example, let's take glass. Now, in the visible spectrum (390<λ<700 nm), glass doesn't really absorb, reflect, or emit light (which is why it's transparent). This is due to a quantum mechanics phenomenon called "quantization." Essentially, electrons and the like can only exist in discrete levels of energy, and nowhere in between (like steps on a staircase—you can be on one step or another, but not between them (at least not without falling)). This is odd, and somewhat counter-intuitive because it doesn't appear to occur at the macroscopic level (our daily lives), but it's completely true (as far as we know). When an object absorbs light, it's because that specific wavelength of light is enough to jump an electron...

Let's begin with a journey through time (and space, if you live any distance from Cambridge). The year is 1909, nearly seventy years since the death of John Dalton, the physicist who pioneered atomic theory. It's a little over ten years since Sir Joseph John Thomson (J.J. Thomson—not to be confused with the other J.J. Thomson, who was a philosopher) discovered the electron, and created the "plum pudding" model of an atom. This model stated that atoms were a more or less homogeneous mix of positively-charged and negatively-charged particles (protons and neutrons respectively). Electrons were the negatively-charged ones, because Ben Franklin said so in the eighteenth century (simplification, and possibly slightly farcical). Atoms were more of less the coolest kid in town when it came to physics, but scientists still didn't really know all that much about them—the "plum-pudding" model was mostly a wild guess.