Seen near Paris on Wednesday 8th February by Denis Joye.
"I was working in my office this morning and looked outside at the fog. As the fog was fading away and sunlight was returning in the fog direction, I felt I was time to go out with my camera and wait for a possible fogbow. I was not disappointed and the scenery was really magic! The fog then quickly dissipated and the fogbow lasted no longer than 2 minutes!" ©Denis Joye, shown with permission.
|The previous OPOD described how the electricity and magnetism equations of Scottish physicist James Clerk Maxwell provided, via the work of Gustav Mie and Ludvig Lorenz, a way to predict exactly the coloured rings seen opposite the sun when it starts to shine through a fog.
The theory does more. It automatically takes account of all the optical processes, reflections, refraction and diffraction. It simultaneously predicts the fogbow often seen outside a glory.
Unlike the glory, fogbow formation can be imagined using a simpler physical model. Light waves enter the small fog droplets, some are internally reflected from the opposite side and they then emerge strongly deflected. The process is identical to the formation of a rainbow except that the light waves are significantly diffracted by the small droplet. The result is a diffuse bow with, at most, pale colours.
The IRIS calculation at left imitates Denis Joye's bow with a dark band just inside the primary bow. To do this the droplets were given a 20% (std.dev.) variation in sizes to blur the supernumeraries. Old fog has a wide distribution of droplet sizes!