The Broken Pencil

A GIF Animation

A common classroom demonstation involves placing a pencil in an upright position in 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. As 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 relative to the position of the pencil above the water - the pencil appears broken. Additionally, the pencil as observed through the water, appears fatter than the pencil as observed above the water. Finally, as the pencil is moved farther and farther towards the edge of the glass, the image of the pencil under the water finally disappears from sight.

Why is this phenomenon observed? Of course, the explanation of this phenomenon involves the refraction of light. But just how does the refraction of light cause the pencil to appear fatter and shifted to the side? The awnser to this question is depicted in the animation at the right.

This page was modified by J.T. Schemm of Newton North High School

Comments and suggestions can be sent by e-mail to J.T. Schemm

This page last updated on 06/12/07