Vanishing Coin Experiment

What you need:

  • A drinking glass with straight sides
  • A coin
  • Water
  • A cup to pour the water
  • A small saucer (can be anything to cover the top of your glass)

Here’s is how to do it:

  1. Place the vase of a clear drinking glass over the coin on a flat surface.
  2. Pour the water into the glass.
  3.  Put the saucer on top of the glass and notice the coin DISAPPEAR!

How did it happen?

Making the penny disappear might seems like a magic, but this optical illusion experiment only works at specific angles.  

We can see the objects around us because light. Light from the Sun or artificial light bounces off objects and enters our eyes.  

Generally, there is little or no refraction as light passes through air. Instead, it travels in a straight line. The angle of refraction of light rays depends on the optical density of the materials. 

In the first part of this experiment, when you place the coin underneath the empty glass, a small amount light from the coin reflects off the sides of the glass, but most of it still refracts through and reaches our eyes. 

However, when the glass is filled with water, the light passes through the dense medium of water that changes angle enough that instead of refracting through the glass, all of the light is reflected inwards. This phenomenon is called internal reflection. It is when the light is 100% reflected inwards.

When the glass is filled all the way to the top with water, the light from the coin at the bottom reflects from side to side in the glass. Therefore, none of it can escape and reach our eyes. It’s completely trapped inside and the coin appears to VANISH!


Vivian Nguyen

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