To make convincing glass, you actually don't mess with the opacity at all. Instead, you get refraction to do all the work for you. You can use the same technique to create drinking glasses, vases, and any kind of curved glass object. Let's take a look at a few reference photos. The cut-glass
BEHOLD THE HUMBLE drinking glass. You've seen a glass like this a hundred times, so you know what it should look like, right? Unfortunately, so does everyone else. This means you can't get away with a cheezy rendering of a transparent tube sitting on a transparent cylinder. A little physics is required to get a realistic rendering.