This is an experiment and an optical illusion! Very easy and quick to realize.
How does it work?
The refraction of light is showing you the coin. Refraction is the change in direction of a wave, here, an optical wave, when the medium changes, here air to water.
To understand, you can imagine a straight line going from the coin to your eye. If you walk back this line will get stuck on the bowl. When you pour water into the bowl, it will divert this line, and you will be able to see the coin!
- A coin
- A bowl
How to make this experiment?
- Put the coin into the bowl so that you can't see it.
- Pour water slowly into the bowl without moving the coin.
- The coin will appear to you!