Formation of Color, Reflection
Location: Outside U.S.
Date: Summer 2012
One the smallest level, the atomic and small level, how are colors formed? Also, how do atoms reflect light?
1. The simple case is electronic transition of atoms and molecules.
A) Energy level: In quantum mechanics, electrons in atoms and molecules can take only on certain discrete energy values which are "energy levels." They can change energy levels by absorbing or emitting a photon of electromagnetic radiation with the same energy to the energy difference between the two levels. Usually the absorption transition starts from the ground state which is the lowest possible energy level, to the excited states which is higher energy levels. (can apply similar to semiconductor crystals in which they have band structures.)
B) In the electromagnetic spectrum, there are many ranges of radiation such as X-ray, ultraviolet, visible light, infrared, microwave, etc (from high to low energy sequence). Electronic transition can occur in all spectral range, but people can only see visible light (400nm-700nm in wavelength). If emission is more efficient in electronic transition, you can actually see the color corresponding to the energy level; however when it is dominant to release energy as nonradiative transition which does not involve emission, absorption process is more dominant to cause counter color of materials. For example, trees absorb red light efficiently to maximize photosynthesis process but their leaves do not emit (as it is a loss of solar energy for trees) so they look green which is a counter color of red. However, if you see laser dye in solution under external light source (which has high quantum efficiency or has dominant radiative process) the color is actually corresponding to the energy level.
Ref. http://en.wikipedia.org/wiki/Energy_level, http://en.wikipedia.org/wiki/Color, http://en.wikipedia.org/wiki/Quantum_yield
2. There are other things such as structural color or thin film interference/diffraction grating but it is beyond your question, I guess. The examples are peacock's tail, oil film on water, and CD surface.
Ref. http://en.wikipedia.org/wiki/Structural_coloration, http://en.wikipedia.org/wiki/Thin-film_interference,
3. Reflection: simply, it is from the strong interaction between light and atoms (and their electrons).
A) When light (electromagnetic wave) reaches to materials, it can penetrate (transmission), be absorbed (absorption), or be reflected/refracted (reflection/refraction). We talked about the 2nd case above where the energy level is matched to the wavelength of light. Thus, when the energy level is not matched to the wavelength of light, either light can penetrate (transmission) through transparent materials, or undergo reflection/refraction for opaque materials.
B) Reflection/refraction: once light reaches to a material, it induces each atom radiates a small secondary wave. The sum of these waves causes reflection and refraction: the reflection is combined from the backward radiation from the surface and the refraction is from the forward radiation. Usually reflection happens when light bounce off a barrier, and refraction happens when it pass from one medium to another.
Weonkyu Koh, Ph. D.
Los Alamos National Lab.
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Update: November 2011