Why do Rolexs cost thousands of dollars when a Casio can tell the time just as accurately? It is the same with almost every product. More expensive does not always equal better quality.
If you had looked at all of the posts where people answered your questions, you will see that a digital signal is made up of 1s and 0s. A signal is either a 1 or a 0. So how does that signal vary with glass? It doesn't. It becomes a 1 or a 0, just like everyone has said.
You can do a search until your heart is content on the subject between glass and plastic optical cables and coax vs. optical for that matter. A lot of people prefer glass to plastic, because they think glass is a more durable and clearer material so the 1s and 0s must be more accurate. A lot of people prefer plastic, because it is not as brittle as glass, so the strands have less chance of breaking. For a 3 foot cable it is not that big of a deal, but for long runs , you may have to use a higher quality materials.
A lot of times there are sarcastic posts here that are not appreciated, but true. When someone asks which digital cable is better. They say, I like the optical or the coax, because the 1s sound cleaner.
A lot of people prefer coax to optical, because of jitter. Some prefer optical, because you do not have to worry about interference.
If an interconnect takes care of the last 3% of the sound in the audio chain, a digital cable will take care of .000005%. Just enjoy the music and don't worry about the minute details.
If you are truly just interested in the science of digital, then post your questions in the sound science forum.
