M4 Light Entertainment I

Background

Two of the number of processes light undergoes when it interacts with matter is reflection and refraction. Depending on the property of the matter it interacts with, light either bounces back (undergoes reflection) or it passes through the matter but its speed and direction may change–referred to as refraction. The consequence of these simple processes can be from just unusual to dramatic in real life.

The Discrepant Event: Now you don’t see it…now you do.

This is the easiest of the discrepant events in the “light entertainment” series. In this demonstration–one that you may have seen elsewhere or in a book–I placed a coin at the bottom of a bowl and had everyone standing around at a distance such that the coin was out of sight. I then slowly poured water into the bowl. However, when performing this particular demonstration, I didn’t have anyone film it. Instead, I took two photos. (Notice that both the photos were taken from approximately the same angle and distance from the bowl.)

coin in the bowl - empty bowl small coin in the bowl - with water 2
 The appearance of the bowl before water is poured into it. The appearance of the bowl after it was filled with water.

And the question is: Why does the coin come into view when the bowl is filled with water?

Concepts involved:

  • image, normal, rare medium, dense medium, reflection, refraction, angle of incidence, angle of refraction, speed of light, line of sight. (Additional advanced concept: refractive index)

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