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.
First, Othello addresses Iago as 'Villain'. At this point in the play, this has far less to do with Othello's suspicions of Iago's treachery, than with his resistance to the idea that Desdemona is unfaithful and thus to anyone who suggests it. In dramatic terms, 'villain' is ironic, since it indicates Othello's state of mind (deeply jealous, even paranoid) as well as Iago's true nature - something so far known to the audience but not to Othello. Therefore, 'be sure...give me the ocular proof' demands actual evidence in order for Othello to justify his jealous fear, and to go on trusting the man he regards as his one incorruptible friend.
More ironically still, what follows is not physically 'ocular' at all, but a play on Othello's fevered imagination: Iago's vivid suggestion of Desdemona and Cassio clasped in passionate embraces, followed by an elaborate story of Cassio's incriminating erotic dream of Desdemona - all of which provides Othello not with 'proof', but with a sufficiently lurid mental picture to convince him. Then, finally, Iago tells Othello that 'ocular proof' exists - he claims to have seen Cassio with the all important handkerchief. By the end of the scene Othello is utterly convinced of Desdemona's betrayal and Iago's fidelity - without any 'ocular proof' at all.
this quote is important because here reader can clearly see the inner self of othello that is in a desire of a proof against desdimona. one can see here the trace of othello's helping hand for iago as its not iago who is in fault of manupulating things but othello also provides him with the space to talk about it.from this quote , one can understand that othello is some how trusting iago. and the biggest reason for can be the society where there was a huge scope for illegitimate relations.