90 The Sky 
raindrop, blue light may predominate and the drop will ap- - 
pear a deep blue; or, if you move the drop to a different posi- 
tion, the drop may appear yellow, green, orange, or red. 
Hence in a cloud of raindrops, all those making a particular 
angle between the direction of the sun’s rays and the direc- 
tion to the observer will appear in one color, and all those at 
RAINBOWS 
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POINT OPPOSITE 
SUN 
a different angle will appear a different color. The figure 
shows how this produces the circular shape of the rainbow, 
and the spectral sequence of colors. 
KINDS OF RAINBOWS 
There are many rainbows theoretically possible, but usu- 
ally only three are visible: the primary, secondary, and ter- 
tiary. The primary bow is by far the brightest. It is opposite 
the sun and has a radius of about 42°. Its outer border is 
