Act 1 scene 3 ends with Iago's soliloquy. Why does he think that this plan may be succesfsful in Othello?
Iago's plan in Act 1, Scene 3 of Othello is brilliant. He understands people very well, and he knows what do to do to instigate conflict. He is quite suspicious of Othello, who he believes is sleeping with Emilia. He is angry with Cassio for taking the position of lieutenant that Iago felt belonged to him. So, Iago needs to take revenge on both Othello and Cassio.
Perfect revenge has several characteristics: the victim must suffer more than the avenger did, the avenger must exact his revenge with impunity, the victim must know that the avenger is causing his suffering, and lastly, the avenger must be present to see his victim suffer. Iago's plan for revenge would satisfy these characteristics nicely with one exception: Iago does not plan to make his victims know that he is the avenger. Iago prefers to catch his victims as a spider traps his prey; they will fall into an invisible web and the more they struggle to extricate themselves, the more ensnared they will become.
Iago's plan is to make it seem as if Desdemona and Cassio are having an affair. Cassio is a "proper," or handsome, man, according to Iago. He is "framed to make women false." A man such as Cassio could easily arouse jealousy in other men. Cassio is in fact young, eloquent in speech, and white--all qualities Othello lacks. Further, Iago knows that Othello trusts him, that Othello is of a "free and open nature" and deems others honest, because he himself is. Iago is confident that Othello will believe him when he suggests that Desdemona is sleeping with Cassio. After all, Othello has known Iago much longer than he has known Desdemona. Why wouldn't Othello believe him?
In this way, Iago can ruin Cassio, for surely Othello will fire him if he thinks he is sleeping with Desdemona, and he can destroy Othello's marriage as he thinks Othello has destroyed his. The plan is brilliant in its simplicity.