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The Magic Test Tube
I walked into a classroom one day and set a beaker of slightly yellow colored fluid on the table. I told my class this was magic fluid and I would show them what it could do.
I took a Pyrex test tube and put it inside a small towel. I used a hammer to smash the test tube into very small pieces. I then poured the small pieces into the beaker.
A few seconds later I reached into the beaker with a pair of tongs and pulled out an intact Pyrex test tube.
What was the magic fluid and how did it help me "reassemble" the test tube?
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Solution
The fluid was Wesson Oil. The index of refraction of Wesson Oil is almost exactly the same as that of Pyrex. When light passes from Wesson Oil to Pyrex, there is no reflection.
I put a test tube in the Wesson Oil before showing it to the class. Since there is no reflection when the light passes from Wesson Oil to Pyrex, the test tube is virtually invisible. After pouring the pieces of the test tube into the beaker, I simply pull out the test tube that was in the Wesson Oil all along.