OPOD - Brocken Spectre & Glory

Last updated on September 16, 2023

OPOD - Brocken Spectre & Glory: Unveiling the Mysteries of Atmospheric Optics

Have you ever witnessed a captivating display of light and color in the sky that left you in awe? One such phenomenon is the Brocken Spectre and Glory, two optical effects that occur when the sun's rays interact with mist or fog. In this article, we will delve into the intricacies of these phenomena, shedding light on their formation and the scientific principles behind them.

The Brocken Spectre: A 3D Shadow in the Mist

When the sun shines through a thin layer of mountain mist, it can create an enchanting visual spectacle known as the Brocken Spectre. This phenomenon is characterized by the appearance of a magnified shadow, seemingly projected onto the mist. The shadow takes on a three-dimensional form, extending from the observer. It is named after the Brocken, a mountain in Germany where it was first documented.

The explanation behind the Brocken Spectre is relatively straightforward. It occurs due to the interplay between the observer, the sunlight, and the mist. As sunlight passes through the mist, it scatters in various directions. Some of these scattered rays reach the observer's eye, creating the illusion of a larger-than-life shadow. The size and shape of the shadow can vary depending on atmospheric conditions and the position of the observer.

Unraveling the Mystery of the Glory

Accompanying the Brocken Spectre, you may also witness another captivating phenomenon known as the glory. This optical effect manifests as a series of colored rings encircling the point directly opposite the sun. Unlike the Brocken Spectre, understanding the formation of the glory is more complex.

The glory arises from a diffraction effect, where waves scattered from different parts of mist droplets combine to produce a colored fringe pattern. These droplets act as scattering objects, dispersing sunlight in various directions. However, the exact mechanism by which the glory forms has puzzled scientists for many years.

Reflection and Polarization: Clues to the Glory's Formation

Through careful analysis, scientists have discovered some intriguing aspects of the glory's formation. By studying the polarization of the glory, they have deduced that waves are reflected inside the mist droplets. This internal reflection plays a crucial role in generating the glory's distinct patterns and colors.

One perplexing aspect of the glory's formation is the path of light that results in its center. Although it may appear that light travels directly opposite the sun to create the glory center, this is not the case. The actual path of light is more intricate, involving a 14.4° gap along the droplet periphery that must somehow be bridged for the glory to emerge as observed.

Surface Waves: Unveiling the Mechanism

To explain this phenomenon, scientists have turned to a process that occurs over short distances in small mist droplets: surface waves. These waves travel along and close to the surface of the droplets, playing a crucial role in shaping the glory's appearance. By utilizing a reformulation of Mie-Lorentz theory developed by Peter Debye, researchers have gained valuable insights into the contributions of different wave reflections within the mist droplets.

According to this theory, the major contribution to the glory comes from waves that are internally reflected once within the droplets. However, there are also lesser contributions from waves that undergo multiple internal reflections, such as 5, 6, or 10 times. The interplay between these reflections gives rise to the intricate patterns and colors observed in the glory phenomenon.

In conclusion, witnessing the Brocken Spectre and Glory can be a truly mesmerizing experience. These atmospheric optical effects showcase the wonders of light and its interaction with mist and fog. While the Brocken Spectre can be easily explained as a 3D shadow projected onto mist, the glory poses a more intricate puzzle. Through the study of reflection, polarization, and the role of surface waves, scientists have made significant strides in unraveling the mysteries behind the glory's formation. Yet, there is still much to explore and discover about these captivating phenomena that continue to inspire awe and wonder in those fortunate enough to witness them.

Brocken Spectre & Glory ~ Imaged by Vincent Favre (Crystal de Givre Photography). ©Vincent Favre, shown with permission.

The sun shines through the thin mountain mist to give two separate optical effects. The first, the 'Brocken Spectre' is easy to explain. It is the 3D shadow of the photographer extending through the mist.

The second, the glory, the coloured rings around the point directly opposite the sun is less easy.

We know that it is a diffraction effect where waves scattered from different parts of an object combine to give a coloured fringe pattern. The scattering objects here are mist droplets.

We know, from the glory's polarisation, that waves are reflected inside the droplets.

But, the ray/wave path 'X' directing light 180° from its original direction to produce the glory centre is impossible. The actual path would be the white one shown.

There is a 14.4° gap along the droplet periphery that somehow must be bridged for light to emerge as it is actually seen.

There is a process only possible over the short distances of small mist droplets where waves travel along and close to the surface ~ surface waves.

Analysis using a reformulation of Mie-Lorentz theory by Peter Debye (see Philip Laven's thorough account) shows that the major glory contribution is from waves internally reflected once. There are lesser contributions from waves internally reflected 5,6 and 10 times.

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Reference Atmospheric Optics

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