Airplane induced iridescence

Airflow Induced Iridescence

Fabiano Diniz saw these iridescent colours over a wing.

"At cruise height a smoke (maybe condensation vapor due to the pressure difference) started to flow over the wing, and it showed faint colors.

The vapor density was directly correlated to the turbulence level. When the turbulence was light the vapor was less dense, and thus the colors were very faint. At some points the airplane shook more and the vapor got more dense, brightening the colors.

The sun was about 7 degrees high, to the right of the photo."

©Fabiano Diniz, shown with permission

Beware!

Airplane window colours can form in a different way. In these images by Martin Izzard the colours are formed by birefringence in the window itself.

Many materials refract the components of polarized light slightly differently. When these refracted waves combine they interfere and produce colours.

The two processes can be distinguished with careful observation with a polarizing filter. If - as at left - the distant scene is coloured and the colours change with polarizing filter angle then window birefringence is very likely the culprit.

The colours are from diffraction of sunlight by individual small droplets. Iridescent clouds shine the same way.

Air is forced to travel faster over the upper convex wing surface. The faster moving air expands and cools. If the humidity is sufficient, water vapour condenses into a fine droplet mist. Air beneath the wing is slowed, compressed and heated.

The freshly formed droplets all have similar histories and therefore sizes. They diffract sunlight to form the iridescent colours.