Flying shadows in water
At the bottom of a sunlit swimmingpool, transient patterns of illumination are seen. Undulations on the water surface act like sets of small lenses, sometimes focusing the light, and sometimes dispersing it. The phenomenon is quite analogous to that originating in air. The patterns are actually no real "shadows" since no light is obscured by the process: it is merely redistributed between brighter and darker areas.
Figure: Sunlight illumination in a pool
of water causes characteristic patterns of light at the bottom. These
patterns, caused by refraction at the undulating water surface, have many
similarities with the flying shadows originating in air.
A refractive-index undulation in the atmosphere acts as a lens, focusing the starlight. The illumination of a screen (= pupil plane) at some distance from such a lens varies from place to place because alternate sections of the lens are converging and diverging. When the turbulence causing the refractive fluctuations is at a great distance from the telescope, the irradiance becomes variable in both space and time. This intensity modulation can be observed in short-exposure images of a telescope mirror illuminated by a bright star, as a system of rapidly moving "shadows". With the unaided eye, such "flying shadows" can be glimpsed during the moments before and after a total solar eclipse, when an uneclipsed solar crescent acts as the light source. Then the "shadows" appear as elongated "bands" because of the anisotropic brightness distribution of the solar crescent. Their motions are determined by wind components at various contributing altitudes. However, in contrast to solar eclipse phenomena, shadow patterns from stars are statistically isotropic.
Figure: Shadow bands moving rapidly across
the face of a house were seen in Sicily during a solar eclipse in 1870
(Codona, Sky & Tel 81, 482, 1991).
High-speed observations in the pupil plane of a telescope reveal the corresponding flying shadows in the light of any brighter star. Very illustrative sequences of such images has been recorded by the Applied Optics group at Imperial College (London):
Figure: Pupil [= telescope main mirror]
image for Alpha Gem, recorded on the 1-meter Jakobus
Kapteyn Telescope on La Palma [1ms exposure].
Movie: High time-resolution imaging of
the same 1-m telescope aperture, recorded with a fast-readout CCD running
at 400 frames per second (Applied Optics group, Imperial College).
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