An object gets its colour by absorbing some wavelength and reflecting a particular wavelength of light. Why do atoms reflect only a specific wavelength and absorb other wavelengths?
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$\begingroup$ my answer to a similar question here might help physics.stackexchange.com/questions/234678/… . Please note that reflections are macroscopic emergent effects of the microscopic interactions on solids and liquids which are much more complicated in structure than just atoms. $\endgroup$– anna vCommented Apr 14, 2017 at 5:28
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An object gets its colour by absorbing some wavelength and reflecting a particular wavelength of light.
So far so good.
Why do atoms reflect only a specific wavelength and absorb other wavelengths?
It is more complicated than that. Objects are not only atoms, they are bound states in a lattice , which means a lot more energy levels than just the atoms and molecules they are composed out of.
When individual photons hit a lattice surface, depending on their energy, i.e. frequency because $ E=h \nu $ , they may do several things.
1)Scatter elastically backwards because their energy was not enough to raise an energy level in the lattice or the molecule or the atom and be absorbed. This will contribute to the color seen of that object
2) scatter inelastically with the surface molecules of the lattice , i.e. raise an energy level in the lattice, or molecule or atom. It will then disappear from the light beam. The deexcitations will be radially distributed and may have different frequencies due to cascades. That frequency will disappear from the reflected beam, and as a result change the color perceived for the object.
3) Compton scatter on a surface electron, and the photon, though possibly refelected will be of lower frequency than the original
1,2,3 are true for both transparent and opaque objects.
For transparent objects there is a 4) the photon goes through interacting elastically because its frequency cannot raise an energy level in the material.
My answer to a similar question here might help.
