Moses receives three successive visions of God: first he sees God in a vision of light at the burning bush (Ex. 3:2); next God is revealed to him through mingled light and darkness, in the "pillar of cloud and fire" which accompanies the people of Israel through the desert (Ex. 13:21); and then finally he meets God in a "non-vision", when he speaks with him in the "thick darkness" at the summit of Mount Sinai (Ex. 20:21).
Kallistos Ware, in the chapter "God as Mystery" from "The Orthodox Way", St Vladimir's Seminary Press: 1979. 13