“To understand this radiation [ cosmic microwave background ], it is easier to begin thinking about the radiation from a very hot gas like that inside a neon light. The same neon… is, at room temperature, utterly transparent… The character of matter in general changes abruptly when it gets heated above 3,000 degrees or so. Below this temperature, matter is electrically neutral… At high temperatures in a neon light, the electrically charged pieces of atoms become unstuck. Frequent and violent collisions break down neutral atoms into electrons and unbalanced nuclei. Matter in this state is called plasma, and it radiates much of its collision energy in the form of light. …a gas of neutral atoms (like air) is virtually transparent. The free [charged] nuclei and electrons of plasma, by contrast, couple to light's electromagnetic fields and absorb it very efficiently. …You …see light only from the borderline layer of neon between opaque plasma and transparent neutral atoms.”

Longing for the Harmonies: Themes and Variations from Modern Physics (1987)