It is quite confusing to understand that how despite being made of solid material, appears transparent. Though, the actual reason is very straight and technical. The main reason behind this is that, the refractive index of glass in very normal and close to the refractive index of light. This gives light waves an open pathway to flow, and not get diffracted.
And hence, we are able to see right through the glass clearly.
However, despite the fact that glass in transparent, it is important to understand that not all glasses are transparent.
Glasses especially the colored ones, do not give light much of scope to penetrate. As a result, the light rays get deviated ad hence, the light waves get diffracted.
I hope this answer helps!😊