July 2020
Spotlight Summary by Dennis K. Killinger
Below the horizon—the physics of extreme visual ranges
Being able to optically photograph a distant horizontal target through the atmosphere, such as a mountain peak that is 530 km (330 miles) away, is almost unbelievable. While this has been experimentally observed a few times over the past 70 years, it has never been fully explained, until now. Prof. Michael Vollmer at the Brandenburg University of Applied Sciences, Germany, has been able to present the detailed optical transmission/scattering equations that explain the optical physics behind these rare observations, and shows that they can occur under the right, simultaneous conditions of light refraction, light scattering, optimal atmospheric conditions and turbulence, and cleverly chosen locations and times (and even maybe luck). Essentially, Prof. Vollmer explains that being able to see a contrasting target that is under the geometrical horizon involves curvature of the light rays involving abnormal refraction due to close to ground inversion layers and thermocline ducting, a clean atmosphere (low humidity, no turbulence), sufficient contrast of target vs. background, and is best seen if just before sunrise (illuminated target against black background).
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Article Information
Below the horizon—the physics of extreme visual ranges
Michael Vollmer
Appl. Opt. 59(21) F11-F19 (2020) View: Abstract | HTML | PDF