Seeing is believing. Or is it? The information we take in with our eyes is entirely dependent upon the light rays that arrive at the lenses and retinas of our eyes—and those light rays can play some complicated tricks on us. It doesn’t take a lot of shadow, smoke, and mirrors to confuse our perceptions. Even an unobstructed tableau in wide open spaces may serve up an image that doesn’t relay the landscape you might expect. The classic example of this is the lost desert wanderer, stumbling across the sand past cactus and disinterested lizards. As the delirium of the heat and thirst spin his brain, he looks into the middle distance and sees….water! It’s not far off—surely a few more minutes of moving forward will get him to that blessed liquid pool. But with each step forward, the pool recedes a step, and he never reaches it. Of course, it’s only a mirage. Few of us have been in such dire straits, but almost everyone has been driving down the highway on a flat, open landscape and seen that mystical pond floating on the tarmac a quarter mile ahead. While you may have a water bottle in the car, you won’t be able to top it off it at that source. No devastating disappointment there, but what the heck is going on with your vision?
Actually, your vision is fine. Mirages can confuse our brains, but they are not optical illusions. What you see is really before you, and it can be recorded on film. But the rays of light coming to you are not taking their usual course. We’re all familiar with the fact that light moves through different translucent materials at different speeds. When light comes through the air and strikes the surface of a lake, it slows down in the water’s higher density, and consequently bends at an angle. Should you seek to acquire a fish dinner from that lake by the use of a sharp projectile, you’d best not aim directly at the fish you see. It won’t be where you see it, because the rays that reflect off it won’t be traveling a straight line back to your eye. If that’s hard to envision, just stick a pencil in a glass of water and observe the offset image as it goes from dry to wet. In the same way, light will bend as it goes from air into the glass of a prism, throwing beautiful rainbows on your wall. So it’s pretty clear that light rays make a quick redirection as they pass through media of different densities which have a well-defined plane of transition. The light that changes speed and direction as it enters glass or water can be similarly affected by air alone. Different densities of air also make for different speeds and therefore altered paths, but air density changes are gradual. Warmer air is less dense than cooler air, but it being a gas, there is mixing. When air density is altered by temperature differences, the boundary between the two temperature regimes is not sharp, so the light is bent more gradually, rather than it being forced to cut a hard-angled turn at a distinct interface.
OK, back to mirages, like that one on a hot desert highway. Sunlight is baking the hell out of the pavement (or maybe it’s baking the hell into it!). The heat is rising off the road and warming the air sitting directly above it. At a certain height above the road, that heat dissipates, so above that super-heated layer you have a mass of air at a distinctly cooler temperature. Not cool, but cooler. It’s the variation that counts. The light passing through such a scene then, is transitioning through media of incrementally different densities, so the light’s progress is curved into a U-shaped path that can be called parabolic. A light ray falling from the sky heads for the ground, but gets bent back upward by the air density change in the very hot air, and arrives at your eye carrying “sky and cloud” information. So an image that is whitish, blueish, and shimmery seems to be coming from the ground. The shimmery part comes from the fact that the whole system is constantly having little fluctuations in temperature and density. There may be a little breeze, too, to disturb the air. Your brain expects light rays to follow a straight line from an object to your eye. The end result is this: Your brain interprets this delightfully scintillating sky-image refraction as a water surface on the ground, and you go mad with thirst….or perhaps just think, “Cool—a mirage!” MIrages are closely identified with hot desert conditions, and they abound there, but they can happen anywhere, even at much cooler temperatures—there just has to be a great enough temperature difference between the surface air layer and the body of air above it.
There’s another quality to the image in a mirage—it’s upside down. It’s a mirror image, you might say, and in fact the word “mirage” comes from the French se mirer: “to be reflected; to see one’s image in a mirror.” And being inverted is just the beginning of the complications. Mirages come in many forms, with images repeated from two to several times, with inverted ones stacked above, below or in between images that are oriented normally. They can be very complex, and the height of their weirdness is the Fata Morgana, wherein objects at the horizon are stretched with amazing distortion into apparent walls, columns and windows that make for an astounding show. It is interesting to note that while mirages are so different from “normal” vision, virtually everything we see at any time is somewhat distorted. We live like fish in water. The atmosphere completely surrounds us, and in its variability, makes all light rays bend a little away from ruler-straight, so that objects are never seen exactly where they truly exist. Most of the time the margin of error, called terrestrial refraction, is unnoticed and makes absolutely no difference to daily living. However, the error is notable to scientists trying to get extremely precise measurements for positional studies of objects both on Earth and in the sky. For those celestial bodies, there is the further distortion of the light rays, called astronomical refraction, as they pass from the vacuum of space to the variable density within the atmosphere. It starts to seem incredible that we can see anything properly at all!
You should know that this brief rundown of mirage types and mechanics is simplified in the extreme, and may not have answered all the questions it raised for you—I didn’t want to speak beyond my own comprehension. The physics involved are very complicated and involve so many factors that even a brief bit of research is guaranteed to have your head reeling, unless you majored in optics in college. The exercise of looking into the subject, however, is a fascinating reminder of the complexity of the physical processes on this planet, and underscores the amazing power of science to help comprehend and explain them. And that’s no mirage.
If you’re looking for desert or highway mirages, there’s no better place to hunt than Arizona! The Benchmark Arizona Road and Recreation Atlas is chock-full of roadways guaranteed to serve up mirages all summer. It’s full of beautiful maps and fantastic travel info, too. You can get one from Maps.com by clicking here:
caption: A classic desert mirage near Primm, Nevada. No hope for the thirsty.
source: Wikimedia Commons: Brocken Inaglory (Public domain)
caption: Cars driving on wet pavement that’s not wet. Note the inverted images below the cars. Like the desert mirage, those are called inferior mirages because they appear below the actual object.
source: Flickr: Michael (CC by 2.0)
caption: Here’s what’s going on. Objects may appear to be above or below their actual position depending upon the position of the air layers. Superior images aren’t necessarily better, just higher.
source: Wikimedia Commons: Ludovica Lorenzelli, DensityDesign Research Lab (CC by SA 4.0 International)
caption: A superior mirage of a distant ship, called a Fata Morgana. Early mariners thought such visions, were caused by sorcery and named the phenomenon after Morgan Le Fay, a sorceress in Arthurian tales.
source: Wikimedia Commons: Brocken Inaglory (CC by SA 3.0)
caption: Fata Morgana over land, across the Gulf of Riga in Latvia. This one is almost an exact mirror image, but they can stretch into strange, distorted shapes.
source: Flickr: Juris Sennikovs (CC by 2.0)