Thursday, April 05, 2007

Colours in the airplane window

While flying from Izmir to Ankara in Turkey on monday the 2nd of April 2007 at 10.15am Selen Ediger wanted to take an aerial shot of the mountains below with my canon ef 10-22mm lens and Hoya multi coated polarizing filter. With naked eye the colors were not visible but when she looked through the camera she saw that the land and the sky was covered with rainbow colors. The contrast and the saturation are adjusted a bit.

What's the origin of this colours?


Harald Edens said...

Hi Claudia,

(I had to delete my first comment because I had an error in the explanation.)

The colors occur because airplane windows are birefringent due to stresses in the forming of the window plastic. The colors are visible when polarized light entering the window are again polarized ("analyzed") by the viewer on the other side and show where stresses in the plastic occur.

With birefringence the index of refraction becomes different for the light polarization vectors parallel and perpendicular to the anisotropy axis. The direction of this axis and the strength of anisotropy differs across the window, giving rise to varying polarization and index of refraction across the window. So you never know ahead of time what pattern of colors you will see when you hold a polarizer in front of it. You can often see the colors already (very weakly so) without a polarizer (look down to ocean or far up, i.e. at a slant angle through the window, to a place in the sky where the incoming light is already polarized). The pattern in the photo you posted is actually quite regular with showing color bands neatly arranged.

Cheap film polarizers show birefringence too when you hold them crossed to one another. You may also see the colors when you look at an iPod screen through polarized sunglasses, or when you hold some kinds of plastic between crossed polarizers.


Harald Edens said...

I guess I did not explain why birefringence makes these colors, and as I have been wondering about this myself in the past I thought that I would write the explanation of it too.

The different indices of refraction cause retardance through the medium (slower velocity) for one of the two polarization modes. So in general these two waves emerge from the medium out of phase with one another. When you look through the medium at various angles, the light path through the medium varies too, and over certain angles the retardance of one polarization through the plastic layer is a half-integer number of wavelengths. Then the two polarization modes for a particular wavelength are either exactly in phase or out of phase and you see that color if you hold a polarizer at either a 45-degree angle (for in-phase) or 135-degree angle (for out-of-phase) between these polarization vectors. In other words, you see that color if the analyzing polarizer lets the two modes interfere constructively with one another after transmission. Since we can't tell the phase of light with our eyes, nor polarization modes, you only see the colors if the incoming light is already polarized, OR if you use a polarizer to "analyze" the modified polarization state. Since usually the incoming light is not much polarized compared to an actual polarizer, the effect is generally not seen in airplanes unless you hold a polarizer in front of the window.

The window in the photo appears to have had a mostly uniform stress vector in it, because the colors are neatly arranged in bands. But often, you see blotches as the vector changes direction or magnitude.

I hope my lengthy explanation helped! And if I made a mistake somewhere, please let me know.


Selen said...

Dear Harald,

Thanks a lot for your informative comments and the information.


Oscar van der Velde said...

I happened to see such colors in the reflection of the sun in the sea when looking through the viewfinder of the camera through the airplane window. I did not use a polarizer and could not notice these colors with my eyes. I thought this was going to be a funky photo with those colors, but the colors in fact did not show up! I used a manual aperture lens, it was already stopped down to f/6.3 or so, so what you see is what you should get. So perhaps my focusing screen is polarized!