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Why is there an absence of the colors brown, black, and white in the rainbow?

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.

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


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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