God is Israel's light, while Isaiah also associates God's saving light with the Servant and the nations.
Psalm 27:1; Isaiah 42:6; Isaiah 49:6; Isaiah 60:1-3 are key backgrounds.
Jesus does not merely point toward divine light; he identifies himself as the world's light and makes following him the way out of darkness into life.
The claim participates in the Prologue's identification of life and light with the divine Word.
The Pharisees challenge Jesus' testimony.