Still, a rainbow shows
that there remains a significant amount of visible light colored indigo
and violet beyond the blue.
response curves for the three types of cones in the human eye
When we look up at the sky, the red cones respond to the
small amount of scattered red light and also less strongly to orange and
yellow wavelengths.
The green cones respond to yellow and the more strongly scattered
green and green-blue wavelengths.
The blue cones are stimulated by colors near blue wavelengths
which are very strongly scattered. [If there were
no indigo and violet in the spectrum the sky would appear blue with a slight
green tinge]. However, the most strongly scattered
indigo and violet wavelengths stimulate the red cones slightly as well
as the blue, which is why these colors appear blue with an added red tinge.
The net effect is that the red and
green cones are stimulated about equally by the light from the sky while
the blue is stimulated more strongly. This
combination accounts for the pale sky blue color.
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