Glory & Brocken at Macchu Picchu
Imaged at the c1400AD Inca site by Pamela Shapiro.
  ©Pamela Shapiro, shown with permission.
Atmospheric
Optics

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Sunlight shining through early morning mountain mist has produced two effects that illustrate the two extremes of optics. One is the glory, the bright spot surrounded by coloured rings. The other is the photographer’s shadow which in a more monstrous or menacing form was named a Brocken Spectre.

Classical ray optics considers light as traveling along narrow lines or rays which can be refracted or reflected but are otherwise straight. The photographer’s shadow, a tunnel of shaded air through the mist, is fully understandable and predicted using classical optics. Classical optics works when light interacts with objects, like photographers, that are considerably larger than its wavelength.

Reduce the size of the object and light no longer neatly travels or shadows along straight lines or cleanly reflects or refracts. Instead we are in the domain of wave optics where light must be recognised as sets of spreading transverse waves that interact in complex and sometimes non intuitive ways with objects and surfaces. The waves scatter – ‘diffract’. They can interfere constructively or destructively to form fringed diffraction patterns of which the glory is one.

The glory is the result of waves scattered almost backwards into the direction from whence it came by tiny mist droplets. It has no counterpart whatsoever in classical ray optics.

Partly for this reason there is no readily understandable explanation or easily visualised physical model of precisely how it forms. Contrast this with another wave scattering effect, a fogbow, which has the rainbow as its classical wave optics equivalent. Attempts to construct a glory using ray optics lead to impossible ray paths. We know that reflection of a sort inside the droplets is involved together with waves traveling on and near their surface but that is about all. This is not to imply that the physics is lacking, mathematical theories describe and predict the glory perfectly but regrettably not in terms that we can grasp visually or mechanically.

Reality is multi-layered and the nano-world of waves and scattering is much removed from our limited macro experiences.