Oh glass, why doth thou maketh mine head ache? Glass can be tricky to convey because, well, that bastard is practically invisible. When I was in England, I visited a glass house (a place where glass is made right before your eyes), and I was amazed. And a lot hot. Glass needs heat to form and, man was I close to that heat source.
Light can refract inside of glass so using that is one great way to depict it in your art. Take a drinking glass, fill it with water, and put a straw or a fork or something in it. Back up a bit and take a look at how the object within seems to bend.
In 3D art, there are even refraction indices for choosing the exact type of glass so the light bends accurately. Amazing stuff, but important to understand. There's a lot of science behind the things we draw, and it's fun and a good thing to learn some of it:) Enjoy the free textures!