Abstract
The inferior mirage from road surfaces is a common phenomenon, which can be easily seen in everyday life. It has been recognized in the literature as a light refraction phenomenon due to the refractive index gradient caused by the temperature gradient in the air strata above the road surfaces. However, it was also suggested that the mirage is just a phenomenon of specular reflection at grazing incidence. Because of the lack of reasonable and quantitative evidence, the generally accepted light refraction theory has not yet been refuted. Here we show some mirror-like reflection images captured from a road surface stretch in Yujiashan North Road, Wuhan, China, when there was no obvious temperature gradient on or above the road, measured on a winter day in December 2009. This provided direct evidence to doubt the temperature induced light refraction mechanism of the inferior mirage. Furthermore, the critical grazing angle of about 0.2° to the road plane where the mirror-like reflection appears could not make the rough surface scatter incident light as a smooth surface according to the Rayleigh criterion. Therefore the phenomenon is a mirrorlike observation effect of scattering from the surface, which cannot be entirely explained by light refraction via air strata. The results demonstrate that the image-formation mechanism and the observer-based-analysis method shown here potentially offer a means of understanding a wide range of scattering phenomena from rough surfaces at grazing angle; for example, the superior mirages of unusual brightness occasionally observed over frozen lakes and the off-specular reflection phenomenon.
Highlights
The inferior mirage from road surfaces is a common phenomenon, which can be seen in everyday life
The mirages were generally attributed to the graded refractive index in the air strata with a graded temperature distribution over a hot road surface, and to this date, this concept has been widely accepted in the literature [1–13]
The phenomenon of image formation, which becomes a mirage seen from rough road surfaces and in deserts, comes from light scattered from an object with an incident angle equal to the reflectance angle
Summary
Some typical mirror-like reflection images on summer and winter days, shown in Figure 1(a) and (c), respectively, were captured from a stretch on Yujiashan North Road (profile in Figure 1(e)), a local road at the foot of Yujiashan Hill in Wuhan, China. We captured clear mirrorlike reflection images on that day (Figure 1(c)) This clearly contradicts the commonly accepted mechanism of the inferior mirage phenomenon caused by the temperature gradient above a road surface. It can be concluded that the inferior mirage seen from a rough road surface is a specular reflection from the surface at a grazing angle, not caused by the refractive index gradient in the air strata over the road. The latter, at best, is a secondary effect
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