PURPOSE: To illustrate a paradox involving Snell's law.

DESCRIPTION: The rays refracting in the horizontal and the vertical planes bend differently when exiting the dish, causing the ball bearing to become distorted when viewed through the top surface of the water.

It apparently is a non-trivial problem to show that the image produced by a horizontal fan of rays is not at the same location as that produced by the more easily handled vertical fan of rays.

SUGGESTIONS: See Question of the Week #100 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.) See Demonstration Reference File for ray drawing and further information.

EQUIPMENT: Plastic tank of water with ball bearing and washer pedestal.

SETUP TIME: 10 minutes.

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