This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all. (ESV)
As other answers show, light is a metaphor, but, as Paul states, things such as light also tell us about God and are more than metaphors:
18 For the wrath of God is revealed from heaven against all ungodliness and unrighteousness of men, who by their unrighteousness suppress the truth. 19 For what can be known about God is plain to them, because God has shown it to them. 20 For his invisible attributes, namely, his eternal power and divine nature, have been clearly perceived, ever since the creation of the world, in the things that have been made. So they are without excuse. (Romans 1)
The created world testifies to God's existence; specifically it demonstrates God's eternal power and divine nature. This did not happen by chance. Just as God created specific things, He created the way in which those things would continue to exist. So if the continuing aspect of the natural world also gives testimony about God, then God created the natural world to ensure that specific testimony was present. In other words, if there are specific things which God calls upon to describe Himself, or more commonly, to illustrate God's point of view about a subject, then God knew that when creating. In effect those things are created object lessons God is able to use with man. The underlying principle is God's omniscience.
For example, Jesus used objects from the natural world to illustrate a point He wanted to teach. Therefore, when He used something from the natural world, these were not something that "happened" to be a good illustrations; rather, they were specifically made to be the example for the point Jesus was making. Jesus often used sheep, and, when one examines the natural behavior of sheep, one finds more than a "good example." The all knowing God knew what He would teach and created that which He could use when He did teach. Very simply, if there is something in the natural world which testifies about God, that is not by chance. It is from God and purposely made with observable properties to give a specific testimony about God.
If God points to light as an example of Himself, there must be aspects of light which are aspects of God. Therefore, it is important to speak correctly about natural light. Nowhere does Scripture say light was created. Rather God spoke and light was. Scripture later explains:
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)
Light is formed; darkness is created. Therefore that which is created has a specific beginning where that which is formed has no created beginning. Light formed reflects God's eternal nature.
Contemporary science has made two interesting discoveries about light which also reflect aspects of God's nature. First, man has discovered that regardless of one's relative position, the measured speed of light does not change. The name given, relativity, misstates the unchanging aspect of God's nature. The speed of light is always the same, because God does change. God made certain there is a physical property of light which does not change because God does not change.
Second, light does not exist in a singular state. Rather, light exists simultaneously as both particle and wave. The wave-particle duality follows what Jesus taught about God:
I and the Father are one (John 10:30)
They are one, yet different. At the same time, they are inseparable. One may perform individual experiments to show one specific nature. Yet the same light can be shown to have the other nature. That which can be seen individually is known to exist simultaneously in the other state which cannot be seen. Therefore, man has discovered physical light has a threefold nature:
- It always exists as a wave
- It always exists as a particle
- It always travels at the same speed
Anyone with a question about the triune nature of God needs only to look at light in the natural world to see that which is true about God is true about light.
God is light. And in Him is no darkness at all. When man demonstrates the particle nature of light, man also sees darkness. Effectively, light obscures the darkness which is present. This darkness is not present in God. In the Old Testament this is called the shekinah glory of God. The New Testament records an incident of light brighter than natural light:
2 And after six days Jesus took with him Peter and James and John, and led them up a high mountain by themselves. And he was transfigured before them, 3 and his clothes became radiant, intensely white, as no one on earth could bleach them. (Mark 9)
Obviously, light is used throughout Scripture as a metaphor for different things such as good, wisdom, right living. At the same, the natural world has been made to given testimony about God. Therefore, when Scripture itself points to the natural world, there must be more than a metaphoric component to that which Scripture points.