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.
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