Objection 1. It would seem that clarity is unbecoming to the glorified body. Because according to Avicenna (Natural. vi, 2), "every luminous body consists of transparent parts." But the parts of a glorified body will not be transparent, since in some of them, such as flesh and bones, earth is predominant. Therefore glorified bodies are not lightsome.
Objection 2. Further, every lightsome body hides one that is behind it; wherefore one luminary behind another is eclipsed, and a flame of fire prevents one seeing what is behind it. But the glorified bodies will not hide that which is within them, for as Gregory says on Job 28:17, "Gold or crystal cannot equal it" (Moral. xviii, 48). "There," that is in the heavenly country, "the grossness of the members will not hide one's mind from another's eyes, and the very harmony of the body will be evident to the bodily sight." Therefore those bodies will not be lightsome.
Objection 3. Further, light and color require a contrary disposition in their subject, since "light is the extreme point of visibility in an indeterminate body; color, in a determinate body" (De Sensu et Sensato iii). But glorified bodies will have color, for as Augustine says (De Civ. Dei xxii, 3), "the body's beauty is harmony of parts with a certain charm of color": and it will be impossible for the glorified bodies to lack beauty. Therefore the glorified bodies will not be lightsome.
Objection 4. Further, if there be clarity in the glorified bodies, it will need to be equal in all the parts of the body, just as all the parts will be equally impassible, subtle and agile. But this is not becoming, since one part has a greater disposition to clarity than another, for instance the eye than the hand, the spirits ["Animalem," as though it were derived from "animus"--the mind. Cf. I-II, 50, 1,3m; I-II, 52, 1,3m] than the bones, the humors than the flesh or nerves. Therefore it would seem unfitting for those bodies to be lightsome.
On the contrary, It is written (Matthew 13:43): "The just shall shine as the sun in the kingdom of their Father," and (Wisdom 3:7): "The just shall shine, and shall run to and fro like sparks among the reeds."
Further, it is written (1 Corinthians 15:43): "It is sown in dishonor, it shall rise in glory," which refers to clarity, as evidenced by the previous context where the glory of the rising bodies is compared to the clarity of the stars. Therefore the bodies of the saints will be lightsome.
I answer that, It is necessary to assert that after the resurrection the bodies of the saints will be lightsome, on account of the authority of Scripture which makes this promise. But the cause of this clarity is ascribed by some to the fifth or heavenly essence, which will then predominate in the human body. Since, however, this is absurd, as we have often remarked (84, 1), it is better to say that this clarity will result from the overflow of the soul's glory into the body. For whatever is received into anything is received not according to the mode of the source whence it flows, but according to the mode of the recipient. Wherefore clarity which in the soul is spiritual is received into the body as corporeal. And consequently according to the greater clarity of the soul by reason of its greater merit, so too will the body differ in clarity, as the Apostle affirms (1 Corinthians 15:41). Thus in the glorified body the glory of the soul will be known, even as through a crystal is known the color of a body contained in a crystal vessel, as Gregory says on Job 28:17, "Gold or crystal cannot equal it."
Reply to Objection 1. Avicenna is speaking of a body that has clarity through the nature of its component parts. It is not thus but rather by merit of virtue that the glorified body will have clarity.
Reply to Objection 2. Gregory compares the glorified body to gold on account of clarity, and to crystal on account of its transparency. Wherefore seemingly we should say that they will be both transparent and lightsome; for that a lightsome body be not transparent is owing to the fact that the clarity of that body results from the density of the lightsome parts, and density is opposed to transparency. Then, however, clarity will result from another cause, as stated above: and the density of the glorified body will not deprive it of transparency, as neither does the density of a crystal deprive crystal.
Some, on the other hand, say that they are compared to crystal, not because they are transparent, but on account of this likeness, for as much as that which is enclosed in crystal is visible, so the glory of the soul enclosed in the glorified body will not be hidden. But the first explanation is better, because it safeguards better the dignity of the glorified body, and is more consistent with the words of Gregory.
Reply to Objection 3. The glory of the body will not destroy nature but will perfect it. Wherefore the body will retain the color due to it by reason of the nature of its component parts, but in addition to this it will have clarity resulting from the soul's glory. Thus we see bodies which have color by their nature aglow with the resplendence of the sun, or from some other cause extrinsic or intrinsic.
Reply to Objection 4. Even as the clarity of glory will overflow from the soul into the body according to the mode of the body, and is there otherwise than in the soul, so again it will overflow into each part of the soul according to the mode of that part. Hence it is not unreasonable that the different parts should have clarity in different ways, according as they are differently disposed thereto by their nature. Nor is there any comparison with the other gifts of the body, for the various parts of the body are not differently disposed in their regard.
Source: Summa Theologica (New Advent)