What you need: 1 mug 1 coin water

How do you perform the experiment?

Put the coin at the bottom of the mug and shift it to its back wall. Now step back as far from the mug that the coin is just hidden by the rim of the mug and you do no longer see the coin. Ask someone to fill the mug with water.

What do you observe? yes no Nothing happens.     The coin becomes visible.     The coin seems to transform.     The coin seems to rise.     The bottom of the mug seems to rise, and with it also the coin.

You can see the experiment in the pictures below.

The left mug is empty. A small tip of the coin is just visible.
The right mug is filled with water. The coin at the bottom of the mug is now completely visible.

In the figure at the right you can see how you can visualize the light rays way from the coin to the eye.

The light rays leaving the coin are bent at the water’s surface (light refraction). Our eye is unaware of this, so we think the rays run straight forward. Therefore, we get the illusion that the coin lies at the extension of the straight line from the coin to our eyes.