This happens in Act IV Scene 1. Ludovico has been sent fromVenice with orders that Othello is to return to Venice leavingCassio in charge in Cyprus. Othello by this point is "horn-mad",obsessed with the belief, implanted so cleverly by Iago, thatDesdemona is having an affair with Cassio. So when Othello hearsDesdemona explain to Ludocivo that Othello and Cassio areestranged, and that she hopes that they will reconcile "for thelove I bear Cassio", Othello hears this as an admission byDesdemona that she is having a love-affair with him. And anadmission made brazenly in public! He starts to get uncontrollablyangry. Then Desdemona says she is glad to hear about Cassio'spromotion. Another admission! thinks Othello. He loses itcompletely and hits her. Desdemona protests that she has donenothing wrong, and Ludovico is aghast at this patently wrongbehaviour and implores Othello to apologise, which he refuses todo.