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J. Perelman
"Entertaining physics". Book 1.
Chapter 8. Reflection and refraction of light

OLD AND NEW ABOUT MIRAGES

Probably everyone knows, what is the physical cause of ordinary Mirage. The burning hot sand of the desert is a mirror properties because adjacent the heated layer of air is less dense than the overlying layers. Inclined beam of light from a very distant object, reaching this air layer, curves in your path so that in the future the following, he again removed from the earth and enters the observer's eye, as if reflected from the mirror at a very large angle of incidence. And the observer, it appears that spread out before him in the desert to the water, reflecting the coastal subjects.


How is the Mirage in the desert.
This figure is usually reproduced in textbooks,
represents the path of the light beam is inclined to the ground utterly cool.

It would be better, however, to say that the heated air layer near the heated soil reflects light like a mirror, and like the water surface, viewed from the depth of the water. It is not a simple reflection, and that the language of physics is called “internal reflection”. This requires that the beam of light entered into the air layers very gently, more gently than shown on our simplified picture; otherwise will not exceed the “maximum angle of incidence of the beam, but without it does not work internal reflection.

We note in passing one point of this theory, which may cause misunderstanding. The stated explanation requires such a location of the air layers, in which the thicker layers would be higher than the less dense. We know, however, that dense, heavy air tends to sink and displace the underlying conductive layer gas up. How can it be that the arrangement of layers of dense and rarefied air, which is necessary for the appearance of a Mirage?

The answer lies in the fact that the desired location of the air layer is not in the still air, and the air in motion. Heated by the ground air layer is not resting on it, and continuously displaced up and immediately replaced with a new layer of heated air. Continuous change makes that hot sand always some adjacent layer of rarefied air, though not the same, but it is indifferent to of the rays.

The kind of Mirage, which we consider, is known since ancient times. In modern meteorology it is called “bottom” Mirage (in contrast to “top”, generated by the reflection of rays of light by the layers of rarefied air of the upper regions of the atmosphere). Most people believe that this classic Mirage can only be observed in the hot air from the southern deserts and does not happen in the more Northern latitudes.

Meanwhile, the bottom Mirage's not uncommon to see in our area. Especially frequent such events in the summer on asphalt and hudsonianum the roads due to the dark color of extremely hot in the sun. The matte surface of the road then it seems from afar as if sprinkled with water and reflects far. The course of light rays at the Mirage shown. With some observation of this phenomenon can be seen not as rare as people think.


Mirage on hudsonianum highway.

There is a sort of Mirage Mirage side , the existence of which usually do not even suspect. It is a reflection from a heated vertical wall. Such a case is described by a French author. Approaching the Fort, fortress, he noticed that the smooth concrete wall of the Fort suddenly shone like a mirror, reflecting the surrounding landscape, the soil, the sky. After another few steps, he noticed the same change on the other wall of the Fort. It seemed that the grey uneven surface suddenly replaced polished. Standing sultry day, and the walls were feisty, and what was the key to their mirror. The figure below shows the location of the walls of the Fort (F and F') and the location of the observer (a and a'). It turned out that the Mirage is observed whenever the wall warms up enough sun even Managed to photograph this phenomenon.


Plan of the Fort, where there has been a Mirage. Wall F seemed to be mirrored from point A, the wall F' - But'

The following figure shows (left) the wall F of the Fort, first Matt, then shiny (right), as a mirror (taken from point A'). On the left picture shows an ordinary grey concrete, which, of course, can not affect standing near the walls the figures of two soldiers. Right - the same wall for the most part acquired mirror-like properties, and the nearest figure of the soldier gives it its symmetric image. Of course, reflects the rays of here is not the surface of the wall, and only the adjacent layer of heated air.


Grey rough wall (on the left) suddenly becomes like a polished reflecting (on the right).

In hot summer days ought to pay attention to the red hot walls of large buildings and look, I found out phenomenon of the Mirage. Without a doubt, with some attention to the number of observed cases Mirage should substantially increase.

Entertaining physics J. Perelman

 




System Orphus

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