light: visible brightness, or a thing that gives it (the sun, daylight, a fire, a lamp, a blaze of glory)
This is the plain, physical meaning. Light is what lets the eye see, whether it is the ordinary brightness of day, the glow of a fire or lamp at night, or the heavenly bodies God set in the sky. The same word names both the brightness itself and the thing that produces it, so a lamp and the daylight are each a phos.
The New Testament also uses this physical word for something far brighter than any earthly source: the overwhelming radiance of a heavenly appearance. When God or the risen Christ breaks in, the scene blazes with a light that outshines the sun. That blaze is still literal light to the people who are blinded by it, even though it carries the weight of God's presence.
This ordinary sense is the soil in which the deeper meaning grows. Because everyone knows what it is to step out of the dark and see, the writers can take the word up and use it for rescue, truth, and God himself.