Primordial Light—But What is Light?
To get an idea of how complex the answer to that question is, search the web for wave-particle duality.

We will keep our definition of light as simple as possible and leave the question of wave-particle duality to the quantum mechanicians. For our purposes, light consists of subatomic particles called photons. Photons are emitted when there is a change in the energy level of an electron, either a free electron or a member of the indeterminate cloud of electrons that surrounds the nucleus of an atom. At the atomic level, such a change in energy levels occurs in a piece of tungsten wire (the filament) when you turn on an incandescent lamp; in the atoms of a gas when you turn on a fluorescent lamp; when you put on a sweater in dry weather; and even when you comb your hair.* The process also occurs in stars, which are the ultimate source of almost all of the visible light in the Universe.

“I want to emphasize that light comes in this form—particles. It is very important to know that light behaves like particles, especially for those of you have gone to school, where you probably learned something about light behaving like waves. I‘m telling you the way it does behave—like particles.” —Richard Feynman

In each instance cited above, electrons are colliding with other electrons and imparting their energy to the electrons that they strike. The electron that receives the additional energy moves into an orbit farther from the nucleus of the atom. That is not the electron’s preferred location, however, because the atom needs to balance its energy and return to the neutral state that it was in before the collision. That state of neutrality is known as the ground state. In order for the atom to return to the ground state the electron that received the additional energy must discharge the additional energy in some manner. An electron cannot manufacture another electron, but it can discharge excess energy in the form of photons. If those photons fall within certain parameters, we call them visible light.

Two characteristics of light that are very important to astronomers are that the path along which a photon is traveling can be altered by passing the photon through various media such as air, water, or the glass in a telescope; and that photons may also have their paths altered by reflecting them from a mirror (such as the mirror in a telescope). These properties of photons, that their paths can be bent and that they can be reflected, are what enable us to build optical devices of every kind, from bifocals to telescopes. In spite of what is going on at a deeper level (the quantum, or subatomic level) telescopes are relatively simple devices; one can build a telescope (grind a lens or a mirror) with the aid of simple instructions without ever having heard of quantum mechanics.

I have been referring to visible light as if there were another kind—invisible light. In fact, most of the light in the Universe is invisible to our eyes. Radio waves in the AM and FM range, microwaves, cell-phone signals, x-rays, visible light, and gamma rays are all forms of light; they are all part of a continuous electromagnetic spectrum and the carrier of all of those forms of light is the photon. The only difference between visible light and other kinds of light is the amount of energy carried by the photons. Low-energy photons carry radio waves. Boost the energy a bit and you have visible light. The lowest-energy visible light photons are those that we see as red. Continue to increase the energy and you will move through all the colors of the spectrum and move back into the invisible region, starting with ultraviolet light. Keep increasing the energy and you have x-rays. Farther up the spectrum are the most energetic photons in the Universe, the gamma rays. Gamma rays are about a million times more energetic than visible light.
Gray Line
If you would like to delve deeply into the subject of photons and the nature of light, read Quantum Electrodynamics, which is a transcription of a series of lectures by the late Nobel laureate physicist Richard Feynman. Feynman had many extraordinary gifts, not the least of which was his ability to make the very complex comprehensible through the use of clear language and great intellectual honesty (if physics can’t provide an answer to why something is the way it is, Feynman tells you so, and tells you that’s just the way it is.) That’s not to say that Quantum Electrodynamics is easy reading, but it is accessible to a scientifically literate adult or advanced high-school student. Reading it will give you a glimpse into the strange beauty that underlies the world we experience. Strange, because quantum mechanics is counterintuitive. Matter at the smallest scales does not behave at all the way we expect it to. Mirrors, you will learn, don’t really reflect light; in spite of what I said above about reflecting light from a telescope mirror, the reality is that nothing reflects light, but photons interact with matter that they strike in such a way as to make it appear that the photons are being reflected, and it’s that appearance that we perceive as reflected light. We are concerned with how we perceive that light behaves, and not with what is really happening. That allows us to say that mirrors reflect light. What really happens is that when a photon strikes a mirror it is absorbed and a different photon is emitted. This “new” photon has pretty much the same characteristics as the photon that was absorbed except that it is traveling in a different direction, thus it appears identical to our eyes, and we see a faithful reflection in the mirror. Even though the photon that reaches our retinas is not the same photon that struck the mirror, the result is the same as it would be if it were the same photon.
* If you can alter the energy state of an atom by combing your hair, you might conclude that it does not require very much force to move the electrons around in atom. You would be right. In fact, in combing your hair you can pull some electrons off the atoms in your comb.