It seems to be a regular money box with a transparent wall. However, when we throw a coin inside, the money somehow disappears.

Thanks to a mirror reflection we get an illusion that we see the whole box inside. But in the middle, a mirror is placed on a diagonal. Such a construction makes the other half of the box invisible and there is where the coin goes.

This magic money box can be used not only to explain the laws of physics but also those of economy. It is a very good model of budgets of many countries.

Inside a cube-shaped money-box a mirror is fitted diagonally. A half of a symmetric solid shape is stuck in the middle of the mirror. In this way, an illusion that we can see the box inside through a transparent wall is created. Such a construction makes the other half of the box invisible and this is where the coin falls. The light reflected by the solid (and dispersed by its surface) is reflected by the mirror and reaches our eyes. It seems to us that the light was sent off by a real object positioned behind the mirror. We call such a 'non-existing' object an image. Since the image seems to be on the other side of the mirror we call it a virtual image.

The picture on the side demonstrates how the image is created by a plane mirror positioned at the angle of 45 degrees to the viewing plane.