The Physics of Fogbows

Quick Overview

Fogbows, or white rainbows, are created by sunlight interacting with tiny water droplets in fog or mist, which are about 100 times smaller than typical raindrops, resulting in a pale, mostly white arc due to the strong diffraction effects caused by the small droplet size, which spreads out the color spectrum beyond visibility.

Key Points: Fogbows are optical phenomena similar to rainbows but formed by sunlight interacting with very small water droplets (fog or mist) instead of larger raindrops. The small size of fog droplets (around 10 micrometers or less, compared to raindrops at 100 micrometers or more) causes significant light diffraction, which washes out the distinct color separation seen in regular rainbows. Regular rainbows exhibit color separation due to refraction and internal reflection at angles around 40{o} (violet) to 42{o} (red), whereas fogbows are seen at an angle of approximately 140{o} (or 40{o} away from the anti-solar point, depending on the definition used for the reflection angle). Diffraction causes the spectral colors from fogbows to overlap significantly, resulting in a faint, often white or pale arc, sometimes called a white rainbow. The extent of color spreading due to diffraction is inversely proportional to the size of the obstacle (or droplet), meaning smaller droplets lead to a wider, less distinct diffraction pattern (0: $\theta \sim 1/d$). Supernumerary bows, which are extra faint, concentric rings inside the primary bow, are caused by the constructive and destructive interference of light waves, a phenomenon that becomes more pronounced and visible with smaller water droplets.

Context: This video explains the physics behind fogbows, which are atmospheric optical phenomena that appear as faint, often white arcs in fog or mist, contrasting them with the familiar, vividly colored primary rainbows seen in rain. The core difference lies in the size of the water particles involved: raindrops for rainbows and much smaller water droplets for fogbows. The explanation relies on principles of refraction, reflection, and crucially, diffraction and interference.