The Broken Pencil-Ray Optics

A general classroom demonstration includes placing a pencil (or similar object) within an upright position within a round glass of water. The pencil is then slowly moved across the middle of the glass from a centered position to an off-center position. Since the pencil is moved across the middle of the glass, an interesting phenomenon is observed. The position of the pencil under the water is shifted associative to the position of the pencil above the water - the pencil appears broken.