The first light is rather puzzling I will admit. I don’t pretend to know what it is but I’ll share some things nonetheless which make sense to me. I hope they are well received for what they are worth.
When I read Genesis 1, I have to think Biblical cosmology. That instantly rules out modern day thinking that the earth was a Big Bang consequence and that it’s inside a galaxy one of countless other galaxies, in a seemingly endless vacuum of space.
The ancients and even up until recently in history believed in a flat circular earth with a firmament dome that kept the waters above and below from coming in and obviously waters within the “snow globe” terrarium were contained within. The sun moon and stars were localized. Basically it wasn’t as vast and endless as modern day space. If occupied a limited defined space.
In verse 2 without getting technical about the Hebrew because I’m merely trying to share an idea not be dogmatic, the waters over the face of the deep to my mind is saying all the material ingredients that will ever be used for Creation are present in the water and are the contents inside the waters. However this matter or these ingredients or these raw materials have not been worked yet, it’s all in raw form, like an unmixed cake batter that has not been cooked. They are just sitting there.
“The earth was without form and void, and darkness was over the face of the deep. And the Spirit of Elohim was hovering over the face of the waters.”
Genesis 1:2 ESV
Whatever we think of the earth in its final form after day six of Creation at this point it was a soup of matter suspended in water. But it seems that in order to make sense of anything and to provide the energy to separate, cook, create, bind, there was something else that was needed, namely LIGHT or energy. So obviously the Creator spoke Light into existence.
Considering even today no one knows what light actually is, though we know how it behaves in some instances like a particle and others like a wave, and we can harness its power, we still don’t know what it is. What we refer to as light is a very tiny portion of the electromagnetic spectrum called the visible spectrum. I feel that this Light was the whole electromagnetic spectrum.
So when it says let there be LIGHT, I believe the electromagnetic spectrum was created, this includes UV light, radio, microwave, gamma, and so on. It was basically introducing energy to the otherwise dark and energy-less pile of matter sitting in water.
Then the Creator using the electromagnetic spectrum or The Light began to separate and create. This light was the energy on which life would function in Biblical Cosmolgy.
Where it gets confusing is
“And Elohim saw that the light was good. And Elohim separated the light from the darkness.”
Genesis 1:4 ESV
We know that darkness was created
“I form light and create darkness; I make well-being and create calamity; I am the Lord, who does all these things.”
Isaiah 45:7 ESV
So there is a distinction between the electromagnetic spectrum or light and it’s absence. He separates the two. They initially don’t mix. Put differently darkness is not needed in Creation
“Elohim called the light Day, and the darkness he called Night. And there was evening and there was morning, the first day.”
Genesis 1:5 ESV
Now try to visualize day as daytime and night time but not with sun and moon and stars but simply splitting the hemisphere (the dome) of Biblical Cosmology into two and putting Light on one side and darkness on the other side. And just to be clear He adds the phrase evening and morning, the first day. This is repeated throughout and therefore indicates that even though there were no markers to determines time yet, the same time elapsed on all days. This doesn’t allow for gap theories.
By separating the Light to one side and calling it day even before the sun was created I think alludes to when and in what conditions Creation was and could be created. It could not be or at least it was not created in Light’s absence but in its presence. I draw from something the Creator Himself says
“We must work the works of him who sent me while it is day; night is coming, when no one can work.”
John 9:4 ESV
While this verse is speaking of spiritual things I would say that all the creating and separating were done in the light or daytime portion of the day and not in its absence.
Obviously by the end of Creation this Light permeated both the day and the night parts of the Cosmolgy but initially Light and darkness were separated.
So that’s my answer, the Light was the electromagnetic spectrum energy needed to cook and work the raw materials to give rise to Creation. Order out of chaos. None of which would be possible without the Light.
In the Hebrew there is a distinction between this Light and the lights created afterwards.