What do you see when your friend bring a sheet of paper on which arrow was drawn behind the empty cylindrical shaped transparent vessel ? Why do you see a diminished image ?
We will see a diminish image of the arrow.
When the vessel is empty, light from the arrow refracts at the curved interface, moves through the glass and enters into air then it again undergoes refraction curved surface of the vessel and comes out into the air. In this way, light travels in two media comes out of the vessel and forms a diminished image.