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Visible colors are caused due to the discreteness of energy levels in molecules and complexes, and scattering.

However, I am not sure how physicists explained color before they understood this (before 1900).

From what I know, they were able to understand the concept of wavelength. I also know that they knew that white light consisted of different "colored light."

So, what was the predominant theory in explaining color?

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  • $\begingroup$ Just a side note: relativistic effects are also needed to explain some aspects of colour for certain materials like say, gold - it's not just QM and scattering. $\endgroup$
    – JamalS
    Commented Feb 22, 2017 at 20:49
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    $\begingroup$ Not all colors are due to discrete energy levels. Quantum mechanics was developed in part in order to explain these situation (emission spectra of gases).Even saying that colors are "caused" by this or that is a little improper. Electromagnetic waves of different frequencies produce sensation of colors due to the specific functioning of our visual system. There are different ways of producing EM waves, emission for individual atoms and molecules is just one of them. $\endgroup$
    – nasu
    Commented Feb 22, 2017 at 20:54
  • $\begingroup$ I think there must have been many different views about color. I recall one by Descartes. He considered color as a property of the medium. When a light ray travels through the aether it imparts rotations to the "aether's spheres" and the different spins is perceived as different colors. $\endgroup$
    – Diracology
    Commented Feb 22, 2017 at 21:34
  • $\begingroup$ See also hsm.stackexchange.com/questions/5637/… $\endgroup$
    – Margaret Friedland
    Commented Feb 23, 2017 at 22:19
  • $\begingroup$ Possible duplicate of How did people explain color before the splitting of white light was discovered? $\endgroup$
    – Conifold
    Commented Feb 23, 2017 at 23:06

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I am sure that there are some good and fairly complete histories of the thinking over time of what light is, and specifically the colors. The following site is a simple and quick summary of the highlights: http://photonterrace.net/en/photon/history/

It goes from Aristotle (white and black combinations for the colors), to Newton identifying refraction and how it splits light into its colors (his theory: particles with different refractivity), to Hyguins and Young determining that it is a wave and explaining refraction due to it and wavelengths, to Maxwell proving that it is electromagnetic and color was freq and equivalently wavelength, to Einstein who identified it as photons in the photoelectric effect - the start of anything quantum.

Of course there was also the study and theories of how the eye saw colors, that different colors affected different retina parts -- that took some time, and it actually was Goethe who first theorized that each person may see colors somewhat differently. Since that's less about physics than about biology I won't add more on that.

I saw elsewhere a spectrum from different substances back before Maxwell, with sharp lines. See it in the link at http://library.si.edu/exhibition/color-in-a-new-light/science

It's from the Smithsonian, see the spectrum shown for Spectrum Analysis, Six Lectures, in 1868. Don't know what the thinking was on sharp lines in the spectrum, maybe you can get that from the Smithsonian and read. What they said in 1868 on the spectral lines. Somewhere there it says it allows one to identify different materials. Not too shabby for 1868.

Only after Bohr atom was there some explanation of discrete lines and jumps and orbitals, leading of course to QM.

So, the theories changed over time, and became more scientific really after Newton.

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  • $\begingroup$ I read the link and the 1868 book is available. It seems that already astronomers were using the spectral lines for identification . In page 139 it leaves to a future Newton to explain how the elements give the spectral lines $\endgroup$
    – anna v
    Commented Feb 23, 2017 at 5:06
  • $\begingroup$ Fantastic. Amazing how long mysteries like this were around and not able to be explained. And some of them are around now, we just are also waiting for the next Newton to figure it out. Thanks for your note. $\endgroup$
    – Bob Bee
    Commented Feb 23, 2017 at 5:15
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Actually Goethe had a very interesting and ususual theory to rival Newton's.

"Unlike his contemporaries, Goethe didn't see darkness as an absence of light, but rather as polar to and interacting with light; colour resulted from this interaction of light and shadow. For Goethe, light is "the simplest most undivided most homogenous being that we know. Confronting it is the darkness" (Letter to Jacobi)."

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  • $\begingroup$ A good example of falsely mystifying the absence of something. Well, they didn't have lights at night other than candles, must have been scary $\endgroup$
    – Bob Bee
    Commented Feb 24, 2017 at 2:33

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