New observations frequently require new explanations and, what is more, they may require the overthrow of theories that have seemed unassailable.
Physicists have to keep an open mind, being prepared to abandon long- standing theories if they no longer satisfactorily explain new observations.
The mirage is a phenomenon that is easily observed in sunny days over plane surfaces highly exposed to sun rays, let us see the explanation that some author give about this phenomenon:
"...Occasionally, when the earth surface is hot by solar radiation, the air layers next to the surface are hotter than the upper layers. It makes an unstable balance of the air density that instead of decrease it increase whith the altitude, until some altitude. In consequent the refraction index also grows with the altitude and the light rays, deflect with the concave to up when they are near the surface. Under this conditions, the light rays coming from the observed objects located near the earth surface could go to the observer eye in a straight line or by a curved line that go near the earth surface. In consequence, the observer will see the image in the real position and other inverted as it was reflected by a water surface. This phenomenon, knows as mirage, is seen frequently in the desert, creating the illusion of water surface. The same phenomenon usually can be seen in hotter roads surfaces..." (1).
"...phenomenon produced by the atmospheric refraction is the mirage. The necessary condition for this is that the air next to the earth surface has to be less dense than the one on the upper layer, situation that is present some times in a place heated intensely by the sun rays..." (2).
(1) "Fundamentos de Optica", Bruno Rossi, page 56
(2) "Optica", Francis W. Sears, page 9
The origin of this definition comes since 1789 by the French mathematics Gaspar Monge interpretation in a Napoleonic African expedition.
The first phenomenon can only be observed when the surface is heated by the sun rays. Similar phenomenon can be observed when we look distant objects over any heated source. The explanation of this phenomenon is that the light rays when cross the space between the object and the observer, suffer different refractions according to the different factors which are affected the air particles, that are mentioned by Jeremy Nam�as:
"...The temperature of the air particles moving in an air mass is affected by the following factors:
The temperature, specially on the earth surface, is so affected by these factors that make impossible consider it as a representative element to identify an air mass after given up his origin region..."(3)1.- The conduction and the mix.
2.- the condention and evaporation.
3.- The expansion and compression (adiabatic changes).
4.- The insulation and radiation.
(3)"Introducci�n al Estudio de las Masa de Aire y al An�lisis Isentr�pico", Jerome Nam�as, page 7.
The second phenomenon as exposed in the next paragraph can be observed during the 24 hours of the day and in every day of the year.
Mirage observation during the 24 hours of the day
The mirage is a phenomenon that observed in a simple view seems distant then with the use of prismatic we can easy look its characteristics, and better, if this observation we make since an altitude closer to the earth surface as possible.
Midday Mirage
The photograph 1 was taken in the city of Tacna south of Peru, on a road towards the east at 12:00 p.m. on may 9, 1976 with a 135 mm lens at 48 centimeters above road surface. The temperature was 31� centigrades at the road surface and 36� centigrades at 90 centimeters over the road surface. Futures controls of the temperatures at this levels show notable variation due to the factors that affect the air particules before mentioned.
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Early Rising Mirage
The photograph 2 was taken at 05:45 a.m. on may 16, 1976 with a 135 mm. lens, at 48 centimeters above road surface. The registered temperatures where 12� centigrades in both thermometers, surface and at 90 cm over the road surface. In this photograph you can clearly observe that the street light, illuminates the surround city atmosphere and this reflects on the road, as if the road is wet; also you can observe the street lights reflected in the road as much as undulation have the road.
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In these photographs we can appreciate that this phenomenon can be observed as in the hours that the earth surface is hotter by the solar radiation than in the hours that they are cooler, that is at early rising.
Night Mirage
When we travel on a road at night, in some occasions we can see that the vehicles light beam comes to us multiply seeming that are coming more vehicles than they are, and this is because that the light beams of those cars are reflected on the road surface as much as undulations have the road.
In order to observe at night or early in the morning the wet impression on the road surface as we easily see in hotter days, that is the sky light that reflect on the earth surface, we must choice a road out of the city giving face to the city, as shown at photograph 2, which is imperative that the observer must be ubicated as closer as possible to the road surface provided with a prismatic, and will see that the light of the street that illuminate the surrounding atmosphere of the city, will be reflected on the road surface giving the impression that this were wet, and also, we can see that the focus will be reflected on the road surface.
Temperature Control at Earth Surface Level and at 50 cm Over
In chart 1 you can observe the average temperature taken by three thermometers, two at road surface level and the other one at 50 centimeters above surface, at the Cuajone mining camp, located in Moquegua - Peru, at 3,500 meters over sea level, between 06:00 and 17:00 hours.
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The illusion that the road seems to be wet, can be also observed on vertical walls, in order to see this phenomenon we must locate one of ours eyes close to a high wall, and to look almost tangent to the wall, and we will see the sky light reflected on the wall.
At figure 1 we can see another way to see the objects reflected on surface that reflect the light diffusely, as a paper sheet, in order to avoid any undulation of the paper, it will be advisable to bend the paper sheet as shown in the picture, and to look almost tangent to the paper sheet any light source, object hardly illuminated by sun rays or the imagen emited by a television set.
In the experiment that shows at figure 1, we can see a similar phenomenon as the mirage that you see on the earth surface, it means the objects reflection, and being that in this experiment the temperature on the paper surface and in the air next to it, has not any intervention, we must conclude for analogy, that we can appreciate in the atmospheric refraction the temperature has not any intervention in the cause of the mirage that sees on the earth surface.
Solar Radiation
This phenomenon is usually better seen when the earth surface is hotter due to solar rays and not counting with the thecnology advances which we count today, is natural that the primary conclusion of this phenomenon was originated by the increment of the earth surface temperature. However during the hours that the earth surface is more exposed to sun radiation, intervene two factors:
The false impression that the earth surface be wet at the photograph 1 is bigger than the photograph 2, this is because the quantity and intensity of the solar rays that illuminate the atmosphere and reflect on the earth surface is more than the quantity and intensity than the light of the street's lights that illuminate the atmosphere on the earth surface at the photograph 2.
- Increment of earth surface temperature.
- Increment of quantity and intensity of solar rays incidence over earth surface.
At largest incident angle will be bigger the illuminated area that reflect light's rays to the observer.
- Incident angle (89�40' approx.).
- Surface roughness grade.
- Quantity and intensity of incident light's rays.
A larger grade of roughness on the surface will require a larger quantity and intensity of incident light rays in order to obtain a mirage phenomenon.
The reflection of light over rough surface which in normal condition will reflect diffusely the light, can be explained if we consider that at a larger incident light angle, a larger surface area will be illuminated; in consequence, a larger surface particles will reflect the incident light towards the observed, as shown in figure 2.
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I will appreciate very much your comments.
In the following address I expose an experiment in order to determine without a doubt the cause that originate this phenomenon.
In the address shown below you can see an explication of the phenomenon of the light deviation when passing near the Earth's Moon.