When presenting an idea to a professor: Be optimistic or realistic I'm trying to convince a professor about my research idea, but when I would start writing that email, I get stuck: should I be optimistic, for example, If we apply my idea, then X, Y and Z could happen (Z could refer to "change the world") but I am not sure enough about X, Y and Z. Or to be realistic, for example, I would say: "Okay, that idea, presumably will lead to x. Thank you".

What I was thinking is if my idea really can lead to X, Y and Z then I will impress that professor, but if my idea cannot (at least in that professor's opinion) lead to anything of what I'm supposing, then he will delete my email immediately. I read a book on how to sell any of your idea or product to people by being optimistic not realistic, but I want to ask if you share the same view. 

As a professor who reads/receives  tons of such emails, what is the most thing that impress you, a realistic talk or an ambitious/optimistic talk?  

As a side note: I'm holding a bachelor degree, and I'm presenting my idea as a working subject in my master study
Be realistic. A seasoned professor has (by necessity) developed a keen BS detector (required to wade through teaching, at very least). A (prospective) student who doesn't know how far they can realistically go will either turn out reckless (and dangerous to the project) or get frustrated when things don't work out, perhaps dropping all (and thus being dangerous to the project). As the professor in turn stands of falls with his successes (and "my PhD student failed me" is no excuse, really), it would make you a risk not worth taking.

Yes, it is nice to show that you know where success in the first step(s) could take you, but they might fail, or turn out unexpected results leading elsewhere.