# How does quantum mechanics explain the color of macroscopic objects?

I am familiar with two explanations for the phenomenon of visible color.

The first is that, for any given object, there are some wavelengths of light it reflects and some it absorbs. The reflected rays reach our eyes and thus determine the color of the object from a human perspective. So if an object appears red under white light, it's because it absorbed light from the green-blue-violet end of the visible spectrum and reflected light from the red end.

However, my understanding of light from the quantum perspective seems to contradict this picture. In quantum mechanics (as I understand it) "reflection" is the process of a photon being absorbed and then re-emitted by an atom. In order to be absorbed by an atom, the photon must have energy corresponding precisely to the difference between two possible electron energy states, meaning only certain discrete wavelengths can be absorbed, and these wavelengths should be exactly the same as those emitted.

How is it that macroscopic objects absorb certain wavelengths and re-emit others given what's happening at the atomic level? What am I missing or misunderstanding here?

• Are you talking about the color that you see or the color as measured by an instrument such as a spectrograph? Commented Mar 20, 2023 at 22:49
• Reflection (mirror) and scattering (more common) are both process where the incoming photon changes direction, i.e. is not absorbed. There is interesting discussion about whether this photon is the original or not ..... but in physics in general it is not proven or disproven ... but it may not matter. For your color question the process is scattering not truly reflection. Commented Mar 21, 2023 at 0:56
• Commented Mar 21, 2023 at 1:57
• It's worth noting that the perceived colour of a macroscopic object is not uniquely determined by its emission spectrum, since illusions can cause objects with the same emission spectra to appear different. Commented Mar 21, 2023 at 2:41
• Side note: The macroscopic description is much older than our knowledge about and understanding of atomic spectra (which strictly speaking means only in gases; fluids, solids, plasma are different species in this respect). So conclusions from the first can be wrong or incomplete in several aspects, while the second provide more details and a surprise every now and then. Commented Mar 21, 2023 at 5:43