Birefringence or why airplane windows colour views

Aircraft Window Colours imaged by Martin Izzard between Britain and Gibraltar. He used a polarising filter on the camera. ©Martin Izzard, shown with permission.

Birefringence in a Plastic Lid visualised by polarised light from a computer monitor. The camera polariser was turned until the bright white monitor screen looked dark. ©Les Cowley.

Surprising colour bands often appear in pictures taken through airplane windows. They are especially intense when using a polarising filter.

There are several keys to what is going on: (1) polarised light, (2) the skylight, (3) the plastic of the window and (4) birefringence.

Polarised light: Light is a transverse electric and magnetic oscillation - a wave. Non polarised light has its oscillating electric component in all the possible directions perpendicular to the wave travel. Plane polarised light is more restricted - its electric component varies in only one direction.

Skylight: The light of the clear blue sky illuminating the landscape and flooding through the airplane window is partially plane polarised. The extent depends on the angle from the sun and is greatest at 90�. See Eva Seidenfaden's series of images.

Window: Airplane windows have at least one layer of thick plastic. The production process, or strains within the window, renders the plastic slightly anisotropic, i.e. its properties differ in different directions. Optically, the window is birefringent or double refracting.

Birefringence: In the simplest type of birefringence the material splits light entering it into two distinct rays which are polarised and refracted differently.

Many crystalline materials are birefringent and their split rays give double images. Iceland Spar is a familiar example obtainable from mineral stores. Less well known is that ice is also slightly birefringent and as a result the everyday sundog has two components that can be separated if some care is taken with polarising filters - see Richard Fleet's excellent sets of images.

The two rays also have their colours dispersed differently. The effects of this are not very apparent until we view the window through a polarising filter. Then we see colour bands. They arise from constructive and destructive interference between the two rays. The two rays passing through the birefringent window plastic have slightly different optical path lengths. On leaving, their wave crests can be in phase and combine to give a bright colour. They can be out of phase giving less or no light. The phase condition depends on the wavelength (colour) and the viewing angle. The result is a series of coloured bands crossing the window and the landscape beyond.

Not flying soon? Simulate polarised skylight with a flat screen computer monitor set to white - an excellent source of polarised light. View or photograph the screen through a polarising filter, some sunglasses will do. If the filter is rotated the screen will go from light to almost dark. The dark condition is when the filter is only accepting light polarised at right angles to the direction of polarization emitted by the screen.

Keep the filter in the dark screen position. Now place some plastic wrapping material or a transparent plastic lid between the screen and filter. Most plastics will appear bright and show colour bands that change as the object is rotated. The colours also change when the plastic is deformed or stretched because strain alters the degree of anisotropy and the birefringence properties.