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The Broken Pencil

A GIF Animation

A common classroom demonstation involves placing a pencil (or similar object) 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 answer to this question is depicted in the animation below.

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For more information on the ray nature of light, visit The Physics Classroom. Specific information is available there on the following topics:

• Refraction and Sight
• The Cause of Refraction
• Optical Density and Light Speed
• The Direction of Bending
• The Secret of the Archer Fish
• Snell's Law
• Ray Tracing and Problem-Solving

Other animations can be seen at the Multimedia Physics Studios. Other useful resources regarding the physics of motion is available through the Glenbrook South Physics Home Page.

This page was created by Tom Henderson of Glenbrook South High School.

Comments and suggestions can be sent by e-mail to Tom Henderson.

This page last updated on 8/11/98.