When a light beam penetrates layers of air of different densities, it is curved and appears to the human eye as reflections. Refelctions occur in particular when light appears to be reflected at the boundary between warm and cold air, creating forms that do not actually exist, or letting objects appear in places where they are not in reality. In the desert, such reflections are also known as mirages. In fact, a mirage is nothing more than the light coming from heaven that is redirected above the earth. It can arise, for example, over strongly heated stone, sand and asphalt surfaces when the incident sunlight is reflected by intense vertical temperature and density gradients.