This verse, containing two clauses, is loaded with significance for both judgment and reconciliation tied together through the motif of darkness, stated explicitly in the first clause and implied in the second.
Luke does not record the significance of the sun being darkened, but the significance is two-fold:
(1) The darkness serves to emphasize or even veil the judgment of God upon Jesus. The judgment of God is so terrible, that when he reveals it, he shrouds it in darkness.
(2) Darkness is, in the OT associated with the judgment of God. It symbolized the remoteness and separateness of God.
The tearing of the temple veil, symbolizes, as Hebrews says, the opening of the presence of God (Heb. 9:3, 8; 10:19f) and may also point to the fact that it is no longer God’s dwelling place (Godet). While some debate the location of this curtain, whether it is the temple at the entrance of the Most Holy Place (Exod. 26:31-33) or the curtain at the entrance to the Holy Place (Exod. 26:36-37), the LXX uses the same word as Luke for the curtains at both locations, the writer of Hebrews places it unmistakably at the entrance to the Most Holy Place in his teachings (Heb. 10:19-22). While passersby and the priests would have seen the tearing of the curtain at the entrance to the Holy Place, the sound of such a thick curtain tearing would be unmistakable to the priests who would most certainly be able to hear it. The distinction carries little difference and the lesson remains the same.
I suggest that the darkness motif continues through the verse like this:
The sun, the world’s source of light is darkened when “the light of the world” is put out by divine judgment. The cross is shrouded in the darkness of judgment just as the veil has shrouded the Most Holy Place in darkness. When the veil is torn, the light shines into the Most Holy Place in anticipation of the dawning of the light of the Resurrection of Jesus Christ who takes believers behind the veil into the presence of God the darkness of whose judgment has dissipated before the Light of the World, Jesus Christ.