Light, to me, means truth revealed. The fact that God knows all can be either a great comfort or a source of great fear, depending on one's relationship with Him.

The Apostle takes God's knowledge of the truth and the revalation of that truth as a source of "full joy" to the one who is fully yielded to Him. That same fact should terrify everyone else, but it is a comfort to know that He fully knows us, and that He will disclose Himself to us one day - and we will "see His face and yet live."

Light also refers often in Scripture to His justice. And again, His justice is awesome joy to those who know His mercy, because it shows us the depth of that mercy when we see how awesome His justice is. A seminary professor once shocked the whole class when he said, "One day some of you will rejoice to see your unsaved mother cast into the lake of fire." On the surface a seemingly heartless and cruel thing to say, but given the thought it deserves - knowing that but for His grace we would all receive His justice rather than mercy, the light of His holiness is comfort to those who are His.

-R