Tuesday, August 23, 2022


Mountain enthusiasts who often go hiking have the possibility, although rare, to be faced with the Spectre of Brocken, one of the most fascinating and evocative optical illusions existing in nature.

The spectrum consists of the enormous magnification of the shadow projected by an observer located at a high point on a layer of clouds located below it. When in the layer of clouds, or fog, there are droplets of water suspended the light of the Sun is refracted creating an aura from the colors of the rainbow, thus creating a breathtaking spectacle.

The Brocken spectrum is an extremely rare illusion that only occurs when weather conditions are perfect. First of all, the observer must be in an elevated place, such as the top of a mountain (in the example of the photo a top of the Apennines) or by plane. The observer must then have under him a layer of clouds (or fog) of the suitable composition, in particular as regards the suspended particles of water. In addition, to create the shadow, the Sun must be very low on the horizon and must backlit the observer.

In most cases, the Brocken Spectrum takes on enormous dimensions. This is due to a prospective effect since, since there are no reference points on the layer of clouds where the spectrum is formed, the brain is not able to assess the distance and therefore considers them much closer than they are in reality. In addition, the spectrum can vary very quickly over time, making the illusion even more striking. This is because of the movement of the cloud layer above which the shadow is projected.

The Brocken Spectre is named after Brocken Mountain, a mountain in Germany. This relief is on average covered by clouds or fog for 300 days a year, which is actually the perfect place to observe the optical illusion. It was here that the phenomenon was first described in Europe. Due to its suspended appearance between reality and fantasy, Brocken’s Spectre has inspired several poets and writers, appearing for example in Goethe’s Faust and Coleridge’s compositions.

Credit: Mattia Orsi. 

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