Iago has been trying to make Othello suspect Desdemona of having an affair with Cassio. Othello is becoming jealous, but does not trust Iago entirely. In one significant speech in Act 3, Scene 3 he tells Iago:
Villain, be sure thou prove my love a whore!
Be sure of it; give me the ocular proof;
Or, by the worth of mine eternal soul,
Thou hadst been better have been born a dog
Than answer my waked wrath!
Desdemona has accidentally lost a handkerchief embroidered with strawberries which Othello gave her as a wedding present. Emilia found it, but it Act 3, Scene 3, Iago gets his wife to give him the handkerchief instead of returning it to her mistress. Iago needs "ocular proof" or he is in extreme danger. He manages to have Cassio find it in his lodging. Cassio gives it to his mistress Bianca, a woman of easy virtue, asking her to make him a copy of it. She becomes angry at his neglect and contempt and angrily returns the handkerchief to Cassio in Act 4, Scene 1, accusing him of getting it from another woman. But Iago has contrived to be present with Othello, who is maddened by the belief that Cassio got it from Desdemona and is now treating it with such disdain.
In Act 3, Scene 4, Othello, who has already been told by Iago that Cassio has his handkerchief, asks Desdemona to let him use it. She does not want to admit that she lost it because she knows it meant a great deal to him, and she is hoping to find it. In this scene Othello explains why he places such value on that one handkerchief.
Did an Egyptian to my mother give.
She was a charmer, and could almost read
The thoughts of people. She told her, while she kept it,
'Twould make her amiable and subdue my father
Entirely to her love; but if she lost it
Or made a gift of it, my father's eye
Should hold her loathed, and his spirits should hunt
After new fancies.
So Iago needs ocular proof. Desdemona loses her handkerchief. Emilia finds it. Iago gets it from his wife and causes Cassio to find it. Cassio gives it to Bianca. Bianca angrily returns it to Cassio, but Othello sees this happen and also overhears Cassio joking about his relationship with Bianca--but thinks he is joking about his relationship with his own wife Desdemona. Convinced by this that Desdemona is unfaithful to him, Othello strangles her.
That handkerchief which I so loved and gave thee
Thou gav'st to Cassio.
No, by my life and soul!
Send for the man and ask him.
Too late, Othello learns the truth from Emilia.
O thou dull Moor, that handkerchief thou speak'st of
I found by fortune, and did give my husband;
For often with a solemn earnestness--
More than indeed belonged to such a trifle--
He begged of me to steal't.
(Act 5, Scene 2)
Some critics have suggested that this is a weakness in the plot because the little handkerchief is flimsy evidence that Desdemona is unfaithful and also because of the grossly fortuitous way in which it was passed from hand to hand. We feel very sorry for Desdemona, but we do not feel sorry for Othello because he looks like a fool.