Over the past few years there have been several viral videos posted online that have demonstrated the so-called “Reversing Arrow Illusion.”1,2 This simple demonstration is conducted by putting a clear glass in front of a piece of paper that has had arrows drawn on it. When the glass is filled with water, the arrows appear to reverse direction. The demonstration is shown in Fig. 1.

When any explanation of the physics is offered in the online posts of this demonstration, usually the explanation discusses the phenomenon of refraction, which causes light to bend, and thus causes the arrows to reverse the direction they are pointing. While this type of description is not wrong, it is incomplete.

To illustrate that the concept of the effects of refraction due to the addition of water alone is not enough, the drinking glass (with a circular base) is replaced with a glass...

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