Ask A Scientist© Physics Archive

Question:  Why is there an absence of the colors brown, black, and white in the
rainbow?
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Answer 1:  OK.  A rainbow is white light seen through a prism (the prism being
the raindrops).  The prism breaks the light into all the colors that make up
white - so you do not see white because white is a combination of colors.
Black is an absence of light, so it is not in the rainbow.  Brown is a
combination of rainbow colors, so you do not see it in the rainbow.
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Answer 2:  The rainbow (red,orange,yellow,green,blue,indigo,violet) occurs
because light (from the sun) gets scattered by the presence of moisture in the
sky.  Those rain (or smaller) drops break white light into its component
colors.  Blackness means as ABSENCE of color, or an absence of light being
reflected from a surface.  We see a blue car as "BLUE"
because light hitting it is mostly absorbed....the only color reflected
is blue (or a combination of blues/green, etc.  depending on the exact
SHADE of blue).  Something appears BLACK if all light hitting it is
absorbed and none is reflected...i.e.  the ABSENCE of color or
reflected light.  You can see that color combinations DO occur in the
rainbow , for example orange (combo, of red and yellow which are
adjacent colors in the rainbow--due to their comparative wavelength
as light.  Red is the longest wavelength of the visible light colors
in the rainbow, violet is the shortest.  A brown color is a color
combination of red, orange and green--those colors are not adjacent
in the visible colors of a rainbow so they do not combine to form
a visible brown.  The colors which normally make up the BROWN color,
however, ARE ALL PRESENT in a rainbow, but are not present in the color
combination we call brown.
By the way, did you ever notice there are always 2 rainbows visible when
present..on is more intense, the other is frequently very very pale, but
is present in opposite color order of its partner! Ric Rupnik 
rickru
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Answer 3:  Actually, the relation between true colors and what we
perceive as colors is very complicated.  There was an article
in "Physics Today" about a year ago that described the perception
of light in detail.  Our perception of something as a particular
color depends on the overall light level, for one thing (colors
look different in very dim light as you may have noticed).  What
a rainbow or a prism does is break down light into "pure"
single wavelengths, and the colors of the rainbow include all
the different possible wavelengths of visible light.  But they
do not include a lot of colors that we perceive, because those
perceived colors are actually combinations of the pure
wavelengths (and in fact, many different combinations of pure
wavelengths can combine to give the same perceived color,
although again this depends on light level and other factors).
So brown, pink, white, and lots of those colors you see
in paint color samples are not visible in rainbows because
they cannot be produced by a pure wavelength - they require at least
two.
Arthur Smith
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