What is light?
Light is one of the first phenomena of which a human being is aware.
It is the first thing a newborn baby discovers.
Light straddles two universes: it is both real and symbolic at the same time.
It lies midway between the material and the spiritual, and forms the most basic and natural tie that binds them.
In modern science, light plays this same fundamental role: in a cosmos governed by relativity, the speed of light is the sole constant, the universal yardstick.
Moreover, everything we see is only a category of light, since it is not the objects themselves that we see, but only their light.
This is true also of the Almighty, the Infinite Light, Ohr Ein Sof: we cannot know the infinite, but only what it emanates, its light.
–Rabbi Adin Steinsaltz