In act 3, scene 3, Iago suggests to Othello that it is suspicious that Desdemona, a white woman, did not try to marry a white man, somebody of "her own clime (and) complexion," but instead chose to marry Othello, a Black man. In seventeenth-century England, white people were considered superior...
In act 3, scene 3, Iago suggests to Othello that it is suspicious that Desdemona, a white woman, did not try to marry a white man, somebody of "her own clime (and) complexion," but instead chose to marry Othello, a Black man. In seventeenth-century England, white people were considered superior to Black people, and so interracial marriages were deemed unnatural. Indeed, Iago suggests that Desdemona's choice to marry Othello, a Black man, points to something "unnatural" in her character. By this, Iago means to imply that Desdemona is not to be trusted. Iago also reminds Othello that, according to the social hierarchy of the time, he, Othello, is inferior to Desdemona because of their respective skin colors. In this way Iago plants seeds of doubt in Othello's mind—doubts about Desdemona's fidelity and doubts about his marriage.
Once Iago leaves the stage, Othello laments that
I am black
And have not those soft parts of conversation
That chamberers have.
From this, we can infer that Othello, at least to some degree, believes the contemporary racist stereotypes about Black people. He believes that because he is Black, he is inferior. This insecurity is what makes him vulnerable to Iago's suggestions. Indeed, it is this fundamental insecurity that Iago manipulates to help him convince Othello that Desdemona could be unfaithful. It is by manipulating this insecurity in Othello that Iago convinces Othello that it would only be natural for Desdemona to be unfaithful and exchange him, a Black man, for a white man like Cassio.
After Othello demands "ocular proof" from Iago, meaning proof that he can see with his own eyes, Iago decides to give him that proof in the form of a handkerchief. He gets hold of a handkerchief that Othello has given to Desdemona, and he plants this handkerchief in Cassio's room. He then engineers a scenario in which Othello sees Cassio with the handkerchief, and from this moment on, Othello is convinced. Othello believes that Desdemona has given this handkerchief to Cassio as a sign of her love for him. This handkerchief is the final proof that Othello needs, and it drives him mad. He loses all of his ability to think rationally, and he becomes all the more easy for Iago to manipulate.