Albert Barnes on Acts 12:7

And, behold, the angel of the Lord. Acts 5:19.

Came upon him. Greek, was present with him; stood near him, επεστη

And a light shined in the prison. Many have supposed that this was lightning. But light, and splendour, and shining apparel, are commonly represented as the accompaniments of the heavenly beings when they visit the earth, Lk 2:9, 24:4. Comp. Mk 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, etc. 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. Comp. Acts 16:26.

(b) "his chains" Acts 16:26