I believe the consensus interpretation of the Fathers to be:
- The "light" that is continually attributed to God's nature is its own concept, which we commonly speak of using "optical" language. God has never been without it. Its absence is spiritual darkness.
- The "light" of Genesis 1:3 refers principally to the creation of certain spiritual things - heaven, the angels, etc.
- Secondarily, this "light" is some sort of primordial energy, which may include the kind of light that we experience in everyday life, but isn't limited to that. (This is not necessarily cleanly distinguishable from the spiritual light of the previous bullet point, though; we with our scientific knowledge want to say that light is just a simple matter of physics, but the Bible was not written in the time of Maxwell.)
In the Bible, light is associated above all with God, as in your quotation from 1 John. This light illuminates us, so that being "in darkness" means being dissociated from God (in a state of sin, deprivation, etc.) and being "in light" is the opposite. Some more examples from the New Testament (NRSV) are John 1 (or actually pretty much anywhere in John):
In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things came into being through him, and without him not one thing came into being. What has come into being in him was life, and the life was the light of all people. The light shines in the darkness, and the darkness did not overcome it. There was a man sent from God, whose name was John. He came as a witness to testify to the light, so that all might believe through him. He himself was not the light, but he came to testify to the light. The true light, which enlightens everyone, was coming into the world.
Ephesians 5:
For once you were darkness, but now in the Lord you are light. Live as children of light— for the fruit of the light is found in all that is good and right and true. Try to find out what is pleasing to the Lord. Take no part in the unfruitful works of darkness, but instead expose them. For it is shameful even to mention what such people do secretly; but everything exposed by the light becomes visible, for everything that becomes visible is light.
and 1 Thessalonians 5:
But you, beloved, are not in darkness, for that day to surprise you like a thief; for you are all children of light and children of the day; we are not of the night or of darkness.
Moreover, heaven, the angels, and the saints are all described in terms of light, for example in the Revelation. It would appear that 1 John 1:5, "God is light", should be talking about this kind of light, as opposed to the electromagnetic phenomenon that resembles it. This interpretation is supported by the writings of the Fathers. For example, Augustine reads the creation of light in Genesis 1 as mainly referring to the creation of the heavenly realm and its angels (City of God 11.7, 9):
Under the name of light the holy city was signified, composed of holy angels and blessed spirits. [...] When God said, Let there be light, and there was light, if we are justified in understanding in this light the creation of the angels, then certainly they were created partakers of the eternal light which is the unchangeable Wisdom of God, by which all things were made, and whom we call the only-begotten Son of God; so that they, being illumined by the Light that created them, might themselves become light and be called Day, in participation of that unchangeable Light and Day which is the Word of God, by whom both themselves and all else were made.
Similar remarks can be found in Basil's Homilies on the Hexaemeron, Chrysostom's Homilies on Genesis, and so on. Ambrose, in Exposition of the Christian Faith 1.13(79), has a particularly relevant comment: "Think not, then, that there was ever a moment of time when God was without wisdom, any more than that there was ever a time when light was without radiance." God could certainly not have been living in spiritual darkness. The heavenly city, created under the name of "light", should be understood as reflecting the glory of God's eternal "light". (The optical metaphor of reflection is very common in these writings.)
It is true that Augustine (et al.) also see in Genesis 1:3 the creation of "ordinary" light, though they are at a loss to explain exactly what it means for light to exist without some light-giving body (only created in verses 14-19). Augustine says: "What kind of light that was, and by what periodic movement it made evening and morning, is beyond the reach of our senses; neither can we understand how it was, and yet must unhesitatingly believe it." (11.7). Basil suggests that the light here created was, at least, a special sort of light of indescribable beauty (2.7). In contemporary speculation, and certainly up to the scholastic period, the light meant some kind of abstract energy, not limited to what we perceive as visible light. It is primordial light, analogous to the primordial, unformed, chaotic matter of Genesis 1:2; writers of this period were strongly influenced by Plato's Timaeus and its account of the progressive formation of all things from "raw" substance, the ylem.
