Gayle Shulist, age 13, of Niagara Falls, Ontario, Canada, for her question:
What causes a mirage?
A mirage is a master magician that fools the eye. It even can fool a camera. For example, we see puddles shining on the road ahead. If we pause to take a photograph, most likely the puddles will appear in the picture. But as we approach, the water disappears and we know that the pavement was dry all the time. We can't say that the mirage did not exist. It did but we did not see what we thought we saw.
Mirages are mysterious because both parties involved in the trickery are invisible. One is ordinary white light, that travels in straight lines. The other is the air, which in this case is arranged in layers of different density. In dense layers the gaseous molecules are crowded close together. In less dense layers there are fewer molecules, spread farther apart.
Normally, straight rays of light bounce from various objects and carry true pictures of the scenery to our eyes. In a mirage, the light rays do a double or sometimes a triple bounce on the way. They shift parts of the scenery from here to there and we see a misplaced picture as a mirage.
As a rule, a layer of extra dense air bends, or refracts light rays downward. A layer of thin, less dense air tends to refract the rays up¬ward. And wherever they go, the refracted rays take their images and transplant them somewhere else. The puddle on the road mirage appears on a warm sunny day, when the hot ground warms a thin layer of air above the surface. This warm air is thin and light and the cooler air above it is much denser.
Things are set for a mirage. The lower layer of warm air refracts the light rays up through denser air above. The angled rays transplant a reflected image of the sky a mirage that looks like a puddle on the road ahead. As we get closer it disappears into thin air, which is where it came from.
An illusion of this sort is called an inferior mirage because the image we see is below the real object. Sometimes realistic details are added upside down. Reflections of trees or an on coming car may appear upside down in the puddle on the road.
In a superior mirage, the image appears above the real object. This usually occurs when the lower air is much denser than the air above. Often this happens over the sea. The contrasting air layers refract the light rays from a ship so that its image is lifted aloft and reversed. Above the real ship we see its mirage, sailing upside down in the sky.
Normally we behold the scenery only as far as the horizon. But a type of mirage called "looming" can reveal objects from beyond the horizon, where they should be out of sight. These images also are created by the refraction of light rays through air layers of different densities. Loom¬ing can make a distant ship look taller, show us a mirage of the sun just before it actually rises, or reveal a distant shoreline from beyond the horizon. In polar regions, combinations of different type mirages create fantastic illusions on and above the frozen white wastelands.