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Q & A: Disappearing penny?

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Most recent answer: 10/20/2010
I am trying to help a physics teacher.... please see clip Why does penny disappear under beaker of water?....
- Christina Wood (age 41)
Lincolnshire IL
Hi Christina,
A wonderful demonstration!  I must confess I scratched my head a bit on this one, but it's explainable by Snell's Law and a bit showmanship on the part of the demonstrator.  In the first part there is an air-water interface and all light rays get bent upward according to Snell's Law. The penny seems to vanish but when the camera pans up so you can look through the top surface of the water the penny reappears.  No problem with that but why does it disappear when viewed through side of the vessel?   It's because the water-air interface on the side is rotated by 900 with respect to the bottom interface.  In this case if the camera is less than certain angle, the light rays can't escape and reach it.   When the penny is inside the water, or wet and touching the bottom, the light rays don't get bent upward.  In this case light rays exiting from the side of the vessel have angles less than the critical angle and pass through. 
This would be easier to visualize if the container had a rectangular shape rather than a cylindrical shape.    The web site explains Snell's Law in gory detail but not the trick.  You have to apply the Law twice in order to get it.


(published on 10/20/2010)

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