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Observing with radio light can provide astronomers with a different
view of the Universe compared to the optical view we are more familiar
with. For example, regardless of the
actual phase of
the Moon, the moon appears as
a featureless
disk when observed in radio light with
the Australian
Square Kilometre Array Pathfinder (ASKAP). However, ASKAP has
another trick up its sleeve - it can "see" both the intensity of the
radio light and
its polarisation
i.e. the orientation of the light. Astonishingly, when we look at the
moon in polarisation it looks like a cosmic soap bubble (see image
above) - why is that? When light reflects off a surface at a critical
angle
(known "Brewster's
angle") it becomes highly polarised. Fishermen exploit this effect
by wearing sunglasses that filter out the polarised sunlight (glare)
reflected off the water's surface, allowing them to more easily see
the fish underneath. In the case of the moon, the orientation of the
polarised light from a point on the moon will depend on the
orientation of the lunar surface at that point as seen from ASKAP's
location. The overall effect is that the orientation of the
polarisation will always point towards the centre of the moon, with
the intensity being greatest at the limb. Since colour was used to
represent the orientation of the polarisation in the image above, we
see this as a rainbow effect along the lunar limb with opposite sides
of the moon having the same colour. Since the effect is strongest at
the limb this gives an overall "soap bubble" effect. (Image credit:
E. Lenc, V.A. Moss, & K.W. Bannister (CSIRO) and D.C. Price
(Curtin/Berkeley))
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