|Algal Corona by Marko Riikonen (site, more algal optics). All images ©Marko Riikonen, shown with permission.|
|Not a reflection of an skyward corona in water but a corona produced by light scattered from algal cells on the surface of the small pool.
Cloud and fog droplets are not alone in being able to produce coronae. The only requirement is that the scattering particles be small and of fairly unifom size. They do not have to be transparent nor spherical.
These particular algal scatterers are Nautococcus. They are globular and the cells in the background image are 7 – 17 micron diameter, ideal for creating coronae. They float half submerged and present a hemispherical shape for light scattering.
Light reflected upwards from the pool surface is scattered by the algae to form the corona.
The corona is a vertically elongated ellipse because the scatters present horizontally elongated shapes to the incoming wavefronts. As the sun increases in altitude the cells present ever more circular cross-sections and Marko, who has made an intensive study of algal scattering, comments “I have noticed that Nautococcus coronas become circular at midday sun elevations”.
More of Marko's algal scattering phenomena will be in later OPODs.