7 And God said, Let there be light. The first word of God created the nature of light; it made darkness vanish, dispelled gloom, illuminated the world, and gave to all beings at the same time a sweet and gracious aspect. The heavens, until then enveloped in darkness, appeared with that beauty which they still present to our eyes. The air was lighted up, or rather made the light circulate mixed with its substance, and, distributing its splendour rapidly in every direction, so dispersed itself to its extreme limits. Up it sprang to the very æther and heaven. In an instant it lighted up the whole extent of the world, the North and the South, the East and the West. For the æther also is such a subtle substance and so transparent that it needs not the space of a moment for light to pass through it. Just as it carries our sight instantaneously to the object of vision, so without the least interval, with a rapidity that thought cannot conceive, it receives these rays of light in its uttermost limits. With light the æther becomes more pleasing and the waters more limpid. These last, not content with receiving its splendour, return it by the reflection of light and in all directions send forth quivering flashes. The divine word gives every object a more cheerful and a more attractive appearance, just as when men in deep sea pour in oil they make the place about them clear. So, with a single word and in one instant, the Creator of all things gave the boon of light to the world.
Let there be light. The order was itself an operation, and a state of things was brought into being, than which man's mind cannot even imagine a pleasanter one for our enjoyment. It must be well understood that when we speak of the voice, of the word, of the command of God, this divine language does not mean to us a sound which escapes from the organs of speech, a collision of air struck by the tongue; it is a simple sign of the will of God, and, if we give it the form of an order, it is only the better to impress the souls whom we instruct.
And God saw the light, that it was good. How can we worthily praise light after the testimony given by the Creator to its goodness? The word, even among us, refers the judgment to the eyes, incapable of raising itself to the idea that the senses have already received. But, if beauty in bodies results from symmetry of parts, and the harmonious appearance of colours, how in a simple and homogeneous essence like light, can this idea of beauty be preserved? Would not the symmetry in light be less shown in its parts than in the pleasure and delight at the sight of it? Such is also the beauty of gold, which it owes not to the happy mingling of its parts, but only to its beautiful colour which has a charm attractive to the eyes.
Thus again, the evening star is the most beautiful of the stars: not that the parts of which it is composed form a harmonious whole; but thanks to the unalloyed and beautiful brightness which meets our eyes. And further, when God proclaimed the goodness of light, it was not in regard to the charm of the eye but as a provision for future advantage, because at that time there were as yet no eyes to judge of its beauty. “And God divided the light from the darkness;” that is to say, God gave them natures incapable of mixing, perpetually in opposition to each other, and put between them the widest space and distance.
Source: Nine Homilies of Hexaemeron (New Advent)