When looking at the sun through the frost patterns on the windshield of my car on May 4, 2014, I saw a well-defined yellow and red corona. The ice crystals on the window pane worked as an obstacle which diffracted the light, which means they deviated it into different directions. So the light waves can reach areas which are blocked out when following the direct way. A diffraction pattern always consists of bright and dark spaces. The dark spaces are those where waves extinct each other, while the bright areas are at the positions where the light waves add each other. Coronae which are really circle-shaped form in grids with periodical gaps as for example in fabric. Irregularly collocated diffracting particles, however, form coronae which change their shape significantly when seen from different angles (1–2).
Author: Claudia Hinz
This photograph was taken by Hans-Jürgen Heyen from Meerbusch. “I discovered the glory effect in August 2012 at the pond of Hugenpoet Castle in Essen-Kettwig. It made me take a closer look at the water because one tends to think that there are mineral oil products on the surface. But there were gigantic carps in the pond and also some big golden fish which looked like koi-carps. And also a beautiful demoiselle flew over the water. And especially this species of dragonflies is very sensitive against environmental pollution. However, what really was there in the water were algae which caused a significant clouding of the water, and this obviously was the reason for the formation of the algae glory.”
These coronae are caused by light diffraction on very small particles. In most cases, they are caused by algae, but there were also such coloured rings observed around pollen which had landed on the water surface. The coronae are caused when a ray of light is split up into partial beams by such a small particle. These partial beams go on into different directions and interfere in the observer´s eye.
Just like pollen coronae, also algae coronae are not always round. When there are unusually shaped algae are involved, also their ring systems on the water surface can be ovally shaped or have bright spots.