If you pass the light from a sodium flash through a prism, you get a pattern very different from the familiar continuous rainbow that Newton elicited… - Frank Wilczek

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.

For many centuries before modern science, and for the first two and a half centuries of modern science, the division of reality into matter and light seemed self-evident. ...As long as the separation between the massive and the massless persisted, a unified description of the physical world could not be achieved.

E = mc² really applies only to isolated bodies at rest. In general, when you have moving bodies, or interacting bodies, energy and mass aren't proportional. E = mc² simply doesn't apply. ...For moving bodies, the correct mass-energy equation is
<math>E=\frac {mc^2} {\sqrt{1-\frac{v^2} {c^2}}}</math>
where <math>v</math> is the velocity. For a body at rest <math>(v=0)</math>, this becomes E = mc². ...we must consider the special case of particles with zero mass... examples include photons, color gluons, and gravitons. If we attempt to put m = 0 and <math>v</math> = c in our general mass-energy equation, both the numerator and denominator on the right-hand-side vanish, and we get the nonsensical relation E = 0/0. The correct result is that the energy of a photon can take any value. ...The energy E of a photon is proportional to the frequency f of the light it represents. ...they are related by the Planck-Einstein-Schrödinger equation E = hf, where h is Plank's constant.