Let A (Pig. 44) be a source of sound from which spherical
waves spread out in every direction, and let the circles drawn
with A as centre be the sections of those spherical wave-fronts
which shew the positions of the surfaces of maximum com-
pression at the moment for which the figure is drawn. The
radius of each of these circles increases by about 1100 feet per
second, if the air has its ordinary temperature, and the
distance between any two consecutive circles measured along
a radius is the wave-length of the sound.
Let MN be a rigid wall and the point where the
perpendicular from A meets the wall. At the moment for
which the figure is drawn the wave-front 6 touches the wall
at 0. A little later the part of the wave-front that is not
hindered by the wall will have come into the position 7, but
the part between G and D will have been reflected towards the
left. The part near reached the wall first and was reflected
first, the parts farther from were reflected a little later,
whilst the parts at G and D have just come into a position to
be reflected. Consequently the wave-front between G and D
will have taken some such shape as the curve GEU ; that is to
say, the wave-front which had the form of the sphere 606' has
now been bulged inwards between G and D, and the bulge
is travelling towards the left. As the original wave-front



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REFLECTION



91



continues to spread outwards from A the bulge will get deeper
and wider, and will occupy the successive positions shewn in
the figure.
