A light source is projected through the illuminating prism, the bottom surface of which is ground (i.e., roughened like a ground-glass joint), so each point on this surface can be thought of as generating light rays traveling in all directions. A detector placed on the back side of the refracting prism would show a light and a dark region.
A light source is projected through a prism with its bottom surface roughened like ground-glass. Each point on this surface acts as a source of light rays traveling in all directions, which are detected by a detector positioned at the back of the prism. This results in a bright and dark region being observed.