Hot Wall Lateral Mirage

Richard Blacquiere captured this figure miraged and mirrored by air heated by a long sunlit wall at Hampton Community Theatre, New Brunswick, Canada.

Richard kept the camera roughly the same distance from the wall while the figure moved sideways from shot to shot.

©Richard Blacquiere
The 35m long wall and, at right, the camera location. A long lens is needed for mirages!

Mirages need long path lengths. The refractive index differences between hot and cold air are so small that ray deviations are tiny.

A cold north facing wall might just produce a superior mirage - a challenge for photographers!
A layer of warm air formed next to the sunlt wall. The quality and clarity of the mirrored figure suggest that the upward air flow was gentle and laminar rather than turbulent.

Refraction across a hypothetical and unphysically sharp cool to warm air boundary. The ray entering the hotter and less dense air is deflected towards the cooler layer.

The more diffuse temperature boundaries of nature behave similarly. Rays curve towards cooler layers.

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Vanishing line:

Two rays from "a" can reach the eye - an almost undeviated ray, shown lightly, and one sharply refracted across the air layers.

Point "b" is similar.

Point "c" is different. Only one ray can reach the eye. Rays closer to the wall than "c" do not reach the eye at all. "c" marks the position of the vanishing line where the 'real' image is cut off by the mirage.

The mirage has symmetry about the vanishing line.



Lateral mirages, as these are called, are similar to the mirages made by hot roads that resemble reflections in water. In lateral mirages the air layers of different temperatures are stacked vertically rather than horizontally and the mirror image is close to (and overlapping) the hot wall.

It is not easy to picture mirage rays because they are almost straight and a scale diagram would be impossibly long and narrow. At lower left is an attempt. Distances from the wall are highly exaggerated to show the ray curvatures.


A ray from "a" that can reach the eye penetrates deeply into the warm/cold air boundary. The ray is refracted across the temperature gradients to curve back outwards from the wall so that it appears to come from "a' ". Rays from "b" reaching the eye do not approach the wall so closely and they are more gently refracted. The refracted rays appear to come from a second mirrored figure close to the wall.

The 'real' figure remains visible via almost undeviated rays from "a" and "b" that do not penetrate into the hot air layers. These are drawn very lightly

The symmetry of the human face and figure make it less than obvious that the image near to the wall is a completely new and mirrored version of the real person. The mirage has even started to overlap and erode the wall, particularly at its top.