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It's funny I cannot answer this question, but I really don't have a very good physical intuition for how color works.

I know that sunlight and (say) light from lightbulb is a superposition of all colors of the EM spectrum. When I pass this "light" into a prism, the longer wavelengths are separated from the shorter wavelengths, resulting in a rainbow.

However, I learned that if something is red, it is absorbing all/most colors except red. Our eyes are seeing the wavelength of red reflected from (say) a red shirt, which is cotton with red dye. So, this red dye is "reflecting" red light, and "absorbing" everything else.

The color black absorbs most colors, while white reflects most colors.

What is going on here physically? When I say "absorb" and "reflect", what does that actually mean in terms of electromagnetism?

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The answer to your question is the obverse of it: we assign a color to an object based on the wavelengths which are reflected to our eyes (or in the case of filters, transmited to our eyes). That means other wavelengths are absorbed. The absorption of wavelengths is based, primarily, on the chemistry of the object.

Red dye applied to cotton cloth is a chemical whose molecules absorb less red light than other wavelengths, hence the red wavelengths are more intense than other wavelengths in comparison to the light from other objects. Similarly for blue, green, yellow, etc objects. Most objects of colors don't absorb all the energy of other wavelengths; they just absorb less of certain wavelengths, and we assign a color name based on the modified mixture reaching our eyes.

In fact, the "colors" surrounding each other can modify our interpretation of what color we see. (Search for "color optical illusions". There are fascinating examples.)

Regarding absorb and reflect: they mean exactly what you think. The energy of an EM wave is taken into a molecular structure and not released as the same wavelength (absorption) or it is released as the same wavelength (reflection or transmission).

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  • $\begingroup$ This is a great answer. "chemical whose molecules absorb less red light than other wavelengths" Do you have any details for how this exactly works? $\endgroup$ – ShanZhengYang Aug 23 '15 at 22:15
  • $\begingroup$ I could say some phrases about "bond length" and "resonance" but, frankly, a good explanation of the details is deeper than I can give off the top of my head. A good chemist or molecular spectroscopy physicist would give a better answer to those details. $\endgroup$ – Bill N Aug 24 '15 at 2:32
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Confusion arises when one has not learned that color perception and frequency associated color are two different things: the first is connected with biology and the second with physics.

Perception of color begins with specialized retinal cells containing pigments with different spectral sensitivities, known as cone cells. In humans, there are three types of cones sensitive to three different spectra, resulting in trichromatic color vision.

Similar to mixing paints, green is made by blue and yellow, the brain attributes colors to combinations of frequencies and not only to a frequency. Thus one sees green when no green frequency is present in the reflected spectrum.

The unique association of frequency to rainbow colors is one way: a given frequency is perceived as the rainbow colors, a one to one association. A given color perceived by the eye is a many frequencies to one color association.

Here is the frequency to color association:

rainbow

Here is a biology article on this

colorpercept

Here one sees the association of many colors to a unique frequency. It is not simple. (rhodopsin is "visual purple: a photosensitive purple-red chromoprotein in the retinal rods that is bleached to visual yellow (all-transretinal) by light, thereby stimulating retinal sensory endings. Lack of rhodopsin results in night blindness. Vitamin A is the primary source of rhodopsin.")

Thus as far as physics goes the atoms absorb some frequencies and reflect others according to the energy levels that can be excited in the atoms of the surface, but the color the eye sees is the result of a build up in the retina, thus seeing one color whereas there are many frequencies.

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