Commentary for Exodus 34:30

וייראו מגשת אליו AND THEY WERE AFRAID TO STEP NIGH UNTO HIM — Come and see how great is the power (influence) of sin! For before they streched forth their hand to sin what does Scripture say? (Exodus 24:17) “And the sight of the glory of the Lord was like devouring fire on the top of the mountain in the eyes of the children of Israel”, — and yet they were not afraid and did not tremble! But after they made the golden calf they recoiled and trembled even at the sight of the rays of glory of Moses! (Sifrei Bamidbar 1:8)

והנה קרן עור פניו, “and behold! the skin of his face had become radiant.” We know of the light which represents wisdom and we know light which emerges out of darkness. The former kind of light is the light G’d employed to create the universe. Concerning that light our sages have said that it preceded the first directive of G’d in the Book of Genesis, i.e. “let there be light.” This light which was employed as a tool in G’d’s work of creation is known as ראשית. Moses attained this light in his heart, and it was this light which separated him from all other prophets making him superior to all of them.
The light which emerges out of darkness is the essence of fire, it is known as אור מוגרש, ”light which has been expelled.” This is the light which was applied to Moses, the light which is here called as radiating from the face of Moses. He acquired this ability to radiate light as a result of obtaining the second set of Tablets whereas he had not achieved this ability when he received the first set of Tablets when he was described as “turning and descending from the mountain” (32,15). During the period when the first set of Tablets was being prepared the people had still been under the spiritually uplifting experience of the revelation, everyone of them having been a party to insights never before granted to man. Now that they were no longer on that spiritual level and G’d had even warned them that “no one will ascend (spiritually) with you” (34,3), they would not have credited Moses with continuing to be on such a level had there not been some visible evidence of that. G’d provided this when he made Moses’ skin emit rays of light.
Another way of looking at the absence of this phenomenon when Moses received the first set of Tablets is that seeing his stature of G’d’s prophet had been established beyond all doubt he had not had any need to provide the Israelites with visible evidence of that. Seeing that in His wisdom G’d had been aware that Moses was going to smash the first Tablets, He had delayed equipping him with this power to emit rays of light until he had been given the second set of Tablets.

והנה קרן עור פניו, “and behold, the skin of his face emitted rays of light;” apparently his forehead reflected some of the brilliance of G–d’s “hand,” which protected him from getting a glimpse of G–d’s essence while he was standing in the cleft of the rock G–d had assigned to him for that purpose.

וייראו מגשת אליו, “they were afraid to approach him.” According to the plain meaning of the verse when they beheld him, they thought that they were looking at an angel. (Compare Torah shleymah by Rabbi Menachem Kasher item 241 on this verse)