Imagine that we stand on any ordinary seaside pier, and watch the
waves rolling in and striking against the colums of the pier. Large
blue waves pay little attension to those columns - they divide right
and left and unite after passing each column, much as a army of
soldiers would if a tree stood in their path: it is almost as though
the columns had not been there. But the short waves and ripples find
the columns of the pier a much more formidable obstacle. When the
short waves impinge on the columns, they are sent back and spread as
new ripples in all directions. The obstical provided by the iron columns
hardly affects the long waves at all, but scatters the short ripples.
We have been watching a sort of working model of the way in which
sunlight struggles through the earth's atmosphere. Between us on
earth and outer space the atmosphere interposes innumerable
obstacles in the form of molecules of air, tiny droplets of water,
and small particles of dust. These are represented by the columns
of the pier.
The waves of the sea represent the sunlight. We know that the
sunlight is a blend of lights of many colors - as we can prove for
ourselves by passing it through a prism, or even through a jug of
water, or as Nature demonstrates to us when she passes it through
the raindrops of a summer shower and produces a rainbow. We also
know that light consists of waves, and that the different colors
of light are produced by waves of different lengths, red light by
long waves and blue light by short waves. The mixture of waves
which constitutes sunlight has to struggle through the obstacles it
meets in the atmosphere, just as the mizture of waves at the
seaside has to struggle past the columns of the pier. And these
obstacles treat the light-waves much as the columns of the pier
treat the waves. The long waves which constitute red light are
hardly affected, but the short waves which constitute blue light
are scattered in all directions.
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