Physics

What are mirages?


A lake surrounded by palm trees in the middle of the desert. This is what is called an oasis. Or rather, it would be an oasis if it wasn't just a mirage. This is how it always happens in cartoons: the tired and thirsty traveler rushes toward that tropical oasis, and only when he is about to dive does the lake, along with all the palm trees, disappear.

True, this kind of mirage is just fiction, but mirages do exist and can make it look like there is no water. Contrary to popular belief, mirages are not a hallucination caused by strong heat. They are a real optical phenomenon that occurs in the atmosphere and can even be photographed.

You also don't have to be in a desert to see a mirage. They happen quite often, for example, on large highways on hot days. From afar, you see the image of a vehicle that appears to be reflected in the asphalt of the road, giving the distinct impression that the asphalt is wet and that the vehicle has been reflected by a puddle. But as you get closer, you notice that the highway is completely dry.

Light deviation

The term mirage comes from the French expression. look at yourself that means look at yourself, see yourself in the mirror. Mirages are formed from a phenomenon called by refractive physicists - which is nothing but the deviation of light rays.

Well, but to understand why the deviation of light forms the mirages, you must first of all understand what our vision is like. We can only see why objects reflect or emit light. It is precisely this light that reaches our eyes that is sent by electrical signals to the brain. Interpreting the signals, the brain shapes objects and so we see things.

The problem (if we can consider this a problem) is that our brain understands that light rays always propagate in a straight line. This would even be true if the rays never suffered any detours along the way. Light bias can occur when lightning strikes through media of different densities, such as water to air, or from cooler to warmer air, or through lenses.

You can easily observe the phenomenon of refraction by placing a pencil inside a glass of water. Leaving it partially submerged, you will notice that the pencil looks broken, which is obviously not true. Another case of refraction is that of a fisherman who sees a fish in the sea and sees it closer to the surface than it is. In these two examples, we see objects in a different position than they actually are. This is because we do not see the light bend; we only see the effects of this fold.

But now back to the mirages! Have you noticed that on the beach, on very sunny days, you see things that are a little shakily distant? The physical phenomenon that makes these images look shaky is the same as the formation of mirages in the desert or on the roads.

Due to the intense heat, a layer of warm air forms near the ground. And this air is less dense than the air in the layer just above, colder. As the rays of light spread faster in the warm air, they bend upward. But, as our brain interprets that light has traveled a straight path, what we see is the image of the object, which may be a palm tree, for example, inverted, as if reflected in puddles on the road, or a Lake in the desert. The water is illusory, but the palm tree and its image are real. This type of mirage is called the inferior mirage.

Ghost Ships

There is another kind of mirage, this rarer, and much more impressive, which is called higher mirages. Unlike the lower mirages, they occur by an inverse temperature distribution, that is, a cooler air layer near the surface and above that a warmer air layer. These mirages are also hard to see around because they are typical of polar regions or very cold water.

Higher mirages make the object look far above what it really is. You might, for example, see a boat floating in the air, or it may look much taller than it actually is. In the case of marine mirages, it is possible to form inverted images of ships which, due to the curvature of the earth, are not yet visible. But also direct and suspended images of the horizon are possible. Maybe that's where the legends of ghost ships come from.

The Guinness Book of Records records the most distant object ever seen through a mirage. The schooner Effie M. Morrissey was halfway between Greenland and Iceland on July 17, 1939, when Captain Robert Barlett spotted the Snaefells Jökull glacier in Iceland, which was supposed to be at a distance of 536 to 560 km. The apparent distance, however, was only 40 to 50 km. If not for the mirage, the glacier could not be seen beyond 150 km. It is now known that several glaciers that were discovered were actually mirages. Amazing, no?

You can see an optical phenomenon similar to higher mirages on any clear sky day. Since the earth's atmosphere is not a homogeneous medium - the higher the altitude, the thinner the air - the atmospheric density decreases from surface to space. This fact causes light from a star to pass through the atmosphere in a non-rectilinear trajectory.

Consequently, when we look at the sun, we see it not in its actual position, but higher than it really is. Therefore, the sun can be seen after sunset and before sunrise, even below the horizon. Also, when the sun or moon is very close to the horizon, the light rays from the bottom edge bend more sharply than the rays from the top edge, making them appear elliptical.

Source: Invivo, Science.