Light is a major subject in the bible. It's mentioned for the first time in Gen. 1:3 and for the last time in Rev. 22:5.
In between those two verses, light is used hundreds of times and in dozens of ways throughout both the Old and the New Testaments.
Defining light isn't easy.
Physicists and scientists from various fields still haven't figured out the fullness of what light is, all that it does, and how to accurately describe it.
To me, trying to define light is similar to defining love. It's one of those realities that is better understood by describing what it does rather than by trying to give it a definition.
The simplest way to describe it is to say that light illuminates or makes visible what is already there. When light is present and doing what it does, that which is there is revealed, unveiled, or made known. In fact, that's how Paul describes what light is in Eph. 5:13.
But making things visible is NOT the end-game purpose of light as God designed it.
Light doing what God designed it to do not only makes it possible to see that which is external to us, it ultimately makes it possible to relate to and interact with that which we now see clearly, in the way that He designed us to do so.
Jesus said that He is the light of the world.
He is God made known, unveiled, revealed. He related to and interacted with the Father, the Spirit and all things that He created in the right way.
When God opened the eyes of our hearts and minds and made Jesus visible to us, we began relating to and interacting with our heavenly Father through Jesus in the way that He originally designed us to do so.
But what we need to wrap our minds and hearts around is the reality that because we now interact with God properly, He also calls us and empowers us to interact with everything else in His creation in the way He designed us to.
Our world and most of the people in it are living in darkness--not interacting with God or anything else properly.
He calls and positions each of His people to be a part of their lives--where ever they live on this planet.
Which is why missionaries go where they go and do what they do.
As missionaries interact with Him, those He has called them to live among, and everything else in the way He designed them to, they model and live out what light-filled life is like, enticing and inviting those that are still in darkness to step out of it and in to His marvelous light.