Tuesday, April 10, 2007

Flag corona

Light diffraction doesn't only originate from aerosols like little water droplets or pollen floating freely in the atmosphere, but also from so-called diffraction gratings. These consist of a large number of equally spaced holes or slits, from which the light rays interfere and form an interference pattern. In this example, which I photographed in the beginning of April, the thick woven fabric of the European flag serves as a diffraction grating and shows a beautiful corona.


Timo Kuhmonen said...

Similar patterns -very colorful ones- I see quite often, in the place where I work. There we have some RF-insulated rooms, and special windows that are blocking RF-radiation. When I have looked the structure, it is a very dense transparent wire mesh. You can easily see through that type of window, but everything looks a bit hazy.

Are we talking about same phenomenon here ?

Les Cowley said...

Corona like patterns could originate from a material that has small holes randomly arranged but well separated from each other. In that case we see the sum of the individual diffraction patterns from each hole - as from droplets in clouds. In fabrics and meshes a more likely mechanism is that the apertures are close together and regularly spaced. The material then acts as a two-dimensional diffraction grating. The far field diffraction pattern results from constructive and destructive interference between wavelets originating from the apertures. In the latter mechanism, the observed pattern will not have perfect radial symmetry because the grating spacing is likely different in different directions.