Glass is known to be a transparent material. But ground glass is opaque and white in colour. Why?

Ground glass is a glass whose surface has been ground to produce a flat and rough surface. Glass is transparent as its surface is even in the molecular level resulting in transmission of light rays falling on it. In ground glass the surface is rough due to microscopic unevenness. So when light rays fall on it, then due to rough surface, they are reflected back in different direction instead of transmitting.

Thus the ground glass is opaque and white in colour.