I summed up Gadamer’s discussion of beauty and light a few days ago, but here is Gadamer himself speaking to the subject ( Truth and Method (Continuum Impacts) 
This means in the first instance that light is necessary for beauty to be beauty, since beauty’s essence is to appear: “without light nothing beautiful can appear, nothing can be beautiful.” In addition, it means “that the beauty of a beautiful thing appears in it as light, as a radiance. It makes itself manifest. In fact the universal mode of being of light is to be reflected in itself in this way. Light is not only the brightness of that on which is shines; by making something visible, it is visible itself, and it is not visible in any other way than by making something else visible.” From this “trivial observation,” we can conclude that “It is actually light that makes visible things into shapes that are both ‘beautiful’ and ‘good.’” This is true of visible beauty, but Gadamer argues that something analogous happens with the apprehension of any beauty. Recognition of intellectual beauty requires not visible light but “the light of the mind,” a notion found in Plato and Aristotle and also in the medieval Christian idea of ” intellectus agens .”
Gadamer sees a close connection between this largely Platonic metaphysics of light and the “Christian doctrine of the word, the verbum creans .”
He says, “The light that causes everything to emerge in such a way that it is evident and comprehensible in itself is the light of the word.” There is a “close relationship” between the “shining forth ( Vorscheinen ) of the beautiful and the evidentness ( das Einleuchtende ) of the understandable.” He finds a source for this connection in Augustine’s interpretation of the creation story: “Augustine notes that light is created before the differentiation of things and the creation of the light-giving heavenly bodies. But he puts special emphasis on the fact that the first creation of heaven and earth takes place without the divine word. Only when light is created does God speak for the first time. Augustine interprets this speech, by mean of which light is commanded and created, as the coming into being of mental light, by means of which the difference among created things is made possible. It is only through light that the formlessness of the first created mass of heaven and earth is rendered capable of being shaped into a multiplicity of forms.”
This is in the background of Gadamer’s own theory that “the multiplicity of what is thought proceeds only from the unity of the word.” And this opens up a hermeneutical ontology that overcomes both substance metaphysics and the “metamorphosis of the concept of substances into the concepts of subjectivity and scientific objectivity.” Specifically, this allows Gadamer to justify his notion that things can be spoken of as “events” and that we can properly attribute activity to them. Things are in their self-presentation, in their radiance as beauty or, to put it differently, as language, as self-expression or self-presentation. Plato was thus correct: Beauty is truth because “The beautiful, the way in which goodness appears, reveals itself in its being: it presents itself. What presents itself in this way is not different from itself in presenting itself. It is not one thing for itself and another for others, nor is it something that exists through something else. Beauty is not a radiance shed on a form from without. Rather, the ontological constitution of the form itself is to be radiant, to present itself in this way.”
Here again, as often in Gadamer, I hear distant echoes of Trinitarian theology: The Father’s self-presentation in His eternally begotten Image through the Spirit, which just is the being of God.