Ask A Scientist Physics Archive

   
name         Clarence L.
status       student
age          16

Question -   I do not understand why red light+blue light makes
magenta. I thought that the resultant color of 2 primary colored lights
would be in between the 2 colors. For example, Take red+green, the color
will be yellow. In the EM spectrum, yellow is in between red and green.
So to make the color magenta, don't you need to mix the color blue with
an ultraviolet light?
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Your inquiry is deceptively simple-sounding; however, you have raised a
question that is far too complicated to answer in the short format available
here. In fact it is an issue that has engaged (bedeviled) both artists and
scientists for centuries. For example, Maxwell (of physics fame)
demonstrated that color images can be created from appropriately filtered
black/white images. And Newton demonstrated not only that "white" light can
be de-composed into the visible spectrum, but that "white" light can be
re-composed from the dispersed visible spectrum. Indeed there are many
causes of color some of which do not depend upon the "mixing" of primary
colors. In addition the selection of "red", "green", "yellow", "blue",
"magenta", "cyan", "white", "black" is far more subtle than it appears at
first sight (pun).  I recommend to you a book on the subject of color
entitled "Bright Earth" by Philip Ball. He traces the invention of color
from its inception on the walls of caves to digital cameras. It is
fascinating reading.

Vince Calder
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Clarence,

The light spectrum, as seen by our eyes, is not in fact a line.  It is a
surface based on combinations of three primary colors.  Many colors we see
can be produced as a single wave:  one wavelength, one frequency.  Many
colors we see cannot be produced this way.  We have three kinds of color
sensors in our eyes.  This is why we have three primary colors.  Ultraviolet
is ultraviolet just because its frequency is higher than our sensors can
notice.  The color magenta stimulates both the "red cones" and the "blue
cones" in your retina.  A combination of red light and blue light will do
exactly the same thing.  As a result, a combination of red and blue light
produces the same effect as magenta light.  We then say that red and blue
produce magenta.  If we had only two kinds of color sensors, then we could
describe our perception of light as a line.


Ken Mellendorf
Math, Science, Engineering
Illinois Central College
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