Coin in the bottom of the cup

2001/07/01 Ares, Felix Iturria: Elhuyar aldizkaria

• Fisika
• Museoak

In this experiment we will see that when passing the ray of light from the air to the water curves. To do this we need a bottle, an opaque cup, 5 pesetas or a 5 cent coin of the pound and a little pasta.

First we make the pasta viewfinder. The piece is a little long, about 3 centimeters high, and we add it in the lid of the bottle. Then make a hole in the middle of the piece.

We will place a coin in the bottom of the cup. To not move when adding water it is convenient to paste the pasta to the cup with a ball.

Place the cup on the side of the bottle. To know the distance to which we are located, we will look through the pasta viewfinder and, being very close, we will see the coin background. Remove the cup little by little until the edge of the bowl prevents us from seeing the coin.

The situation is that when looking through the viewfinder placed on the bottle cap we will not see the coin (see figure 1).

Then, when filling the cup of water and looking again, we will realize that the coin looks great.

What has happened?

With the empty cup, the light moves directly. And the edge of the cup does not allow us to see the coin (see figure 2).

When filling the cup of water, the light is refracted, so we see perfectly the bottom and the coin (see figure 3).

If something nice arises about it, we could present it as a magic exchange to our friends and say: “Do you know that water can make a bottom coin appear?”

See if you are successful with the experiment!