Othello's demand for ocular proof, that is, tangible evidence of his wife's adultery, is the turning point in the play. It places Iago in a perilous position. He has to produce physical evidence to support all his false accusations or die. Othello tells him:
If thou dost slander her and torture me,
Never pray more; abandon all remorse;
On horror's head horrors accumulate,
Do deeds to make heaven weep, all earth amazed,
For nothing canst thou to damnation add
Greater than that.
Out of sheer desperation, Iago gets possession of Othello's handkerchief, manages to transfer to Cassio's possession, then contrives to have Othello see Cassio toying with it with Bianca. It is this handkerchief that leads to Desdemona's death, but it is also this handkerchief that leads to Iago's exposure and downfall. Emilia proclaims to many witnesses that Iago had her steal the handkerchief (the ocular proof) from Desdemona and give it to him.
In Act 5, Scene 2, as Othello is about to murder Desdemona, he refers to the ocular proof of the handkerchief.
By heaven, I saw my handkerchief in's hand!
O perjured woman! thou dost stone my heart,
And make me call what I intend to do
A murder, which I thought a sacrifice.
I saw the handkerchief.