In this “disappearing” coin magic trick an example of the effects of light refraction can be seen taking place. In this video a side by side comparison can be seen between the two cups and how the positioning of the coin allows for the trick to happen. At the start of the video both the coins are visible (despite the placement of the coin) through the glass. As the water is poured into the cup the medium changes from air to water and so does the refractive index. The cup on the left can’t be seen because it goes through air to glass to water and the light bends around the coin to mask it, but on the right cup the light only travels through water to glass so it can be see, in these cases the refractive index is whats causing the “magic” trick to occur.