The Broken Pencil

A common classroom demonstration 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.

Anim'n of Light Rays

For more information on the ray nature of light, visit The Physics Classroom Tutorial. Detailed 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

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