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].
Flying-shadow
movie (MPEG, 1 MB)
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).
Updated JD 2,455,775
