In a trivia game, the question was "which color is the only monochromatic light?" I was confused. I believed all colors of the spectrum were monochromatic. The answer was purple. I read this article and still believe the answer purple is wrong. Please correct me, and let me know why purple is the only monochromatic light.
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I agree with you that the answer is wrong. However, the right answer depends on your definition of color.
If you define color as a light of particular wavelength, then you are right - all colors are monochromatic. Except, of course, black (no light) and white (all the frequencies of light we can perceive).
On the other hand if you define color as a property of light perceived by the human eye (which is a bad definition in my opinion), then there is no monochromatic light. I'll give you a few examples to prove this.
If you ask an artist what the primary colors are they will most likely answer that the primary colors are red, yellow and blue, which many of us learn at school.
Your monitor most likely uses standard additive color mixing RGB (red, green, blue), though some modern TVs use RGBY (red, green, blue, yellow).
Most printers use CMYK color model (cyan, magenta, yellow, key-black) which is type of subtractive color mixing.
There are many other color models, including YCbCr (television) and HSV (image editing). All of those color models are capable of producing what appear like perfect images to our human eyes by using only three or four primary colors. The reason that works is because we have two types of cells in our eyes: rods (night vision) and cones (color). There are three types of cones: S, M and L for short, medium and long wavelengths so our eyes can easily be tricked (show us red and green and we see yellow). Some animals have only two types of cones, e.g. dogs, some have only rods, e.g. cows, while other animals have more types of cones than we do, e.g. the mantis shrimp (who has 12 different types of cells for seeing color plus 4 for color filtering). So the image you are now seeing on your monitor would look pathetic to a mantis shrimp, worse than black and white looks to us.
I have another problem with the trivia question that is worth noting. What people think of when they talk about particular color varies across culture, language, dialect, time, where they live... Even if all of those factors are the same there can still be significant difference. See http://en.wikipedia.org/wiki/Purple
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