Observe reflection and refraction at a boundary layer.
Mirages occur when two air masses of different temperatures are up against each other.
While it's difficult to observe reflection or refraction at the boundary layer between densities of air, we can easily see it between air and the heavier medium of water. Notice that some of the light is reflected back so that the surface looks like a mirror. This is especially noticeable if the light source you are using is above the container. Notice that the boundary layer from below also reflects light. Take a pencil or straw and put it into the container at an angle.