Jealousy is the central theme in Othello. Identify those instances where it is mentioned or where it occurs in the play. Prove or disprove by...

The following exchange between Desdemona, Iago, and Othello, concerning Cassio is an example of the jealousy theme that runs through the play.

"Des. Well, do your discretion. [Exit CASSIO. 
Iago. Ha! I like not that. 
Oth. What dost thou say? 
Iago. Nothing, my lord: or if—I know not
what. 
Oth. Was not that Cassio parted from my
wife? 
Iago. Cassio, my lord? No, sure, I cannot
think it 
That he would steal away so guilty-like,
Seeing you coming.
Oth. I do believe 'twas he. 
Des. How now, my lord! 
I have been talking with a suitor here, 
A man that languishes in your displeasure.
Oth. Who is't you mean? 
Des. Why, your lieutenant, Cassio. Good
my lord,
If I have any grace or power to move you, 
His present reconciliation take;
For if he be not one that truly loves you, 
That errs in ignorance and not in cunning, " (Shakespeare)

In this scene, Cassio is speaking to Desdemona, but not because they are having an illicit affair, but because he wants her help to get back into Othello's good graces.  Iago uses the opportunity to interpret the scene as something that it is not, a lover, Cassio, hastily leaving his beloved, Desdemona, in a hurry because her husband is arriving.

Iago makes it appear that Cassio and Desdemona were involved in some intimate activity, which causes Othello to feel suspicion and jealousy.

"Iago, the agency of human evil, is able to twist the distinction between what something is and what it appears to be, and it is this deception that stands at the bottom of Othello's tragic tale."

Iago does not let go of this suspicion, he is the central manipulator in the play who uses Othello's insecurities against him and conjures images that cause the main character to experience raging jealousy.

In the following scene, it is a continuation of the same thought, Iago is trying to stir Othello's imagination against him, since he has to leave his beautiful wife.

"Iago. My noble lord,—
Oth. What dost thou say, Iago?
Iago. Did Michael Cassio, when you woo'd
my lady, 
Know of your love?
Oth. He did, from first to last: why dost thou 
ask? 
Iago. But for a satisfaction of my thought;
No further harm.
Oth. Why of thy thought, Iago?
Iago. I did not think he had been acquainted 
with her.
Oth. O! yes; and went between us very oft. 
Iago. Indeed!
Oth. Indeed! ay, indeed; discern'st thou
aught in that? 
Is he not honest?
Iago. Honest, my lord?
Oth. Honest! ay, honest.
Iago. My lord, for aught I know. 
Oth. What dost thou think?
Iago. Think, my lord!

Oth. Think, my lord!" (Shakespeare)

Iago is a master at setting up Othello to see what is not there to alter his perception of reality.  Iago has a hidden agenda to destroy Othello, so he leads him down a path that he knows will stir his insecurities and create an insurmountable jealousy and rage towards his beautiful wife, Desdemona.