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I am puzzled by light.
SUBTRACTIVE - Red and green pigments, when mixed, appear gray.
ADDITIVE - Red and green photons of light, when mixed, appear yellow.

If red paint reflects red photons and green paint reflects green photons, why does the paint appear gray?  

Is red paint reflecting some red photons while absorbing some red photons?

Also, are all red photons the same wavelength, or are some red photons lighter or darker with different wavelength?


OK, paint doesn't work that way.  When you mix some red and green paints, you get brown.  When you use light, you excite the long and medium cones in the eye in a way that appears yellow, but is not.  Only by separating the light coming from a substance in a spectrometer can you tell what the "real" color spectrum of something is.  Red is a range of wavelengths that humans have deemed "red."  They are not "darker" or "lighter," because they are photons.  You perhaps mean closer to infrared or closer to orange (another arbitrary distinction).  In some languages there are only two or three words for color.  The first two are always black and white, the next is always red, after that are common colors in nature like green and blue.  In China, they split their rainbow into more colors than we do in English.  That's just a matter of perception.  If you mix red and green paint to get gray, then you are observing the paint mostly absorbing light at all of those respective wavelengths (but not completely, or you'd get black). Reflection is really complex, if you try different paints you will get different results.  Your perception is based on the stimulation levels of the three different color cones in your eye.  That's why you can have yellow and blue photons enter your eye (stimulating the short and medium cones), and perceive green when there are no photons in the "green" wavelengths present.

In short, the sky is not just blue...but we see it that way.


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Dr. Stephen O. Nelson


I can answer most basic physics questions, physics questions about science fiction and everyday observations of physics, etc. I'm also usually good for science fair advice (I'm the regional science fair director). I do not answer homework problems. I will occasionally point out where a homework solution went wrong, though. I'm usually good at explaining odd observations that seem counterintuitive, energy science, nuclear physics, nuclear astrophysics, and alternative theories of physics are my specialties.


I was a physics professor at the University of Texas of the Permian Basin, research in nuclear technology and nuclear astrophysics. My travelling science show saw over 20,000 students of all ages. I taught physics, nuclear chemistry, radiation safety, vacuum technology, and answer tons of questions as I tour schools encouraging students to consider careers in science. I moved on to a non-academic job with more research just recently.

Ph. D. from Duke University in physics, research in nuclear astrophysics reactions, gamma-ray astronomy technology, and advanced nuclear reactors.

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