|Mackinac Bridge Mirage
David N Hart imaged the Mackinac Bridge in Michigan, USA with its central 1158m roadway and cables mirrored. Approach sections of the roadway are rendered invisible by the mirage. Images ©David N Hart, shown with permission.
|Warm air beneath cooler air is responsible for this "inferior mirage", named inferior because the mirage image is below the view normally seen.
Light rays from the bridge are refracted as they traverse the temperature gradients between the horizontal layers of air.
The extent of refraction is often quite small and in that event the miraging is only evident close to the horizon and when the object is at some distance. David Hart was about 16 miles away between Cheboygan and Bois Blanc Island on Lake Huron. He was about 5.5.ft above the water level. Mirages can alter dramatically with height.
This mirage shows the 'vanishing line' effect very well. The bridge is mirrored at a level slightly below the central span roadway. Everything below that has vanished. The true horizon is invisible having been replaced by miraged sky. The apparent very choppy ‘horizon’ is purely the lower edge of the mirage.
The ray diagram at left grossly exaggerates the vertical scale and ray curvature in order to portray the effects.
Two rays from the upper bridge (A) reach the eye. The upper ray is slightly curved downwards towards the warmer air. The lower ray is sharply curved and appears to the eye to come from a reflection in the water. Rays from lower down (B) do the same. Position (C) is different. Only one ray reaches the eye. Rays from beneath (C) cannot reach the eye at all - that part of the bridge and water is invisible.
The level (C) is the level of the mirage "vanishing-line". The 'real' view and inverted mirage view join at the vanishing line and the eye can see no details of the scene that are below it. The more distant the object, the higher is the vanishing line. In this mirage seen from 16 miles away the vanishing line is quite high.