Atmospheric
Optics
Wyoming Fogbow
~ A dramatic sighting by John Speier as he approached the Grand Tetons. "I combined two images using an app on my phone as the iPhone cannot capture the whole bow at once."
Image ©John Speier, shown with permission
The fogbow is indeed large, some 80° diameter. Remember if attempting to catch it whole not to use a polarizing filter. Use a polarizer to capture supernumeraries though.
Sunlight making a fogbow follows the same recipe as that for a rainbow (84 degrees across at its red outer rim). Light enters a water sphere, reflects once from the opposite side and leaves in several directions almost back towards the sun. The difference is that large (1-2mm diameter) raindrops give rainbows while small fog droplets (0.01 - 0.2mm diameter) produce fogbows.
The smaller droplets are only 10-100 times the size of wavelengths of visible light and wave effects are therefore significant. At each surface the light waves are scattered in many directions and these outgoing scattered waves overlap and interfere - in other parlance they are diffracted. The result is a broad and diffuse circle of light rather than the sharper rainbow. However, rainbows themselves are not innocent of diffraction effects - witness their supernumerary fringes.
