MOTION MOUNTAIN

152 4 images and the eye – optics
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light
beam
air
sugar and water
F I G U R E 98 Diluted sugar syrup bends light (© Jennifer Nierer).
F I G U R E 97 Light usually travels in a straight
line. In the figure, a sodium frequency laser
beam is used as laser guide star to provide a
signal for adaptive optics in large telescopes.
The laser illuminates a layer of sodium found
in the atmosphere at around 90 km of
altitude, thus providing an artificial star. The
artificial star is used to improve the image
quality of the telescope through adaptive
optics. In the photograph, the images of the
real stars are blurred because of the long
exposure time of 3 min (photo by Paul Hirst).
These systems use a wavelength of13.5 nm, at which lenses are not available. Collimatinganexpandingbeamthusrequiresmanyconcentricmirrors,asshowninFigure96.
Theseopticalsystemsarethe very best that modern technology can provide; for example,
the mirrors have a surface roughness below0.4 nm. Similar optical mirror systems are
also used inx-raysatellitetelescopes.
Does light always travel in a straight line? – Refraction
Usually light moves in straight lines. A laser in a misty night shows this most clearly, as
illustrated in Figure 97. But any laser pointer in the mist is equally fascinating. Indeed, we
use light todefine ‘straightness’, as we explained in the exploration of relativity. However,
there are a number of situations in which light does not travel in a straight line, and every
expert on motion should know them.
In diluted sugar syrup, light beams curve, as shown in Figure 98.The reason is that in
such an experiment, the sugar concentration changes with depth. Are you able to explain
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net