The glories: how to explain this rare optical phenomenon?

Optics is a fascinating science. This branch of physics which deals with light and vision makes it possible in particular to explain certain phenomena which seem to belong to the domain of the supernatural. Among these phenomena is glory, a rare and fascinating optical manifestation.

What is a glory?

When approaching the clouds during a mountain hike or when traveling on an airplane, one can sometimes observe an ellipse surrounding a material object and displaying the colors of the rainbow. As you might expect, this is not a supernatural manifestation, but a phenomenon optical rare known as glory. Although they look very much like circular rainbows, glories are different phenomena from rainbows, especially in the physical processes that produce them.

Being essentially an optical phenomenon, the light of the Sun – and rarely moon – is one of the conditions for the appearance of glory. First, you should know that a glory is always located opposite the sun or the moon. As we already know, light can interact with water droplets in mist and clouds to create different optical phenomena. In the case of glory, this interaction occurs through backscattering or deflection of light by water droplets.

But this is not enough. For a glory to be observed, the sun and the observer must be in some sort of alignment with each other. This alignment is notably defined by the antisolar point, that is to say the point which is directly opposite the sun with respect to the place where the observer is. In other words, the antisolar points are relative to the observer. It is for this reason that it is sometimes possible that in a group of people located in a restricted area, some will be able to observe the glory and others will not.

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This was particularly the case when the French explorer Pierre Bouguer and his companions went on an expedition to the Peruvian Andes in the 1730s, and reported one of the first scientific observations of a glory. The explorers had reported that as they climbed a mountain, they saw the sun emerge from behind a cloud and illuminate them, casting each man’s shadow and surrounding their head with what looked like a halo. In an account of this astonishing discovery, it was noted that each explorer could only see their own halo, and not that of others.

Glory
© Brocken Inaglory / Wikimedia Commons

The explanation of glory by theories of wave optics

While backscattering of light through water droplets provides an explanation for the sighting of a glory, the exact physical theory behind this phenomenon remains a mystery to this day. Over time, several ideas have been put forward to explain this phenomenon and the dominant theory is that which was proposed by the physicist Moysés Nussenzveig in 1987. To explain the glory, he made particular reference to Mie’s solution, which describes elastic scattering of a wave.

Based on this solution, Nussenzveig then argued that a glory was the result of the tunneling effect of electromagnetic waves in the drops of water. More specifically, this means that when sunlight doesn’t hit the water droplets directly, but just passes by, it stirs up electromagnetic waves that eventually work their way out of the droplet and send the light waves back. at their origin. This creates a tunnel effect in which the wave remains trapped in a sphere.

The glories: how to explain this rare optical phenomenon?