Why is ground glass clean and shiny when wet with water?
Answer: Ground glass is rough. Therefore, the light incident on the ground glass is diffusely reflected. That is why ground glass looks opaque. A thin lining falls on the ground glass when wetted with water. As a result, the rough surface becomes slightly smoothed. This causes some of the incident light to pass through the glass, making the glass appear transparent. Again, some portion of the incident light is regularly reflected from the glass surface. This is why rubbed glass looks shiny.