And, behold, the angel of the Lord - See the notes on Acts 5:19.
Came upon him - Greek: was present with him; stood near him ἐπέστη epestē.
And a light shined in the prison - Many have supposed that this was lightning. But light, and splendor, and shining apparel are commonly represented as the accompaniments of the heavenly beings when they visit the earth, Luke 2:9; Luke 24:4; compare Mark 9:3. It is highly probable that this light was discerned only by Peter; and it would be to him an undoubted proof of the divine interposition in his behalf.
And he smote Peter on the side - This was, doubtless, a gentle blow or stroke to arouse him from sleep.
And his chains ... - This could have been only by divine power. No natural means were used, or could have been used without arousing the guard. It is a sublime expression of the ease with which God can deliver from danger, and rescue his friends. Compare Acts 16:26.