Why do atoms emit or absorb light of specific wavelengths?

Question

Why do atoms emit or absorb light of specific wavelengths?

Answer 1

This phenomenon is the result of the quantum nature of matter.  Strictly speaking, Quantum Physics,and Quantum Electrodynamics in particular, are models that predict these kinds of phenomena rather than explain them.  But the basic idea is that a wave function is associated with any bit of matter (including the electrons in atoms).  These wave-functions behave according to rules that imply that electrons in atoms can exist only in certain energy states (called "levels").  When light is absorbed by atoms, the energy in the photon goes to move the electron to a higher-energy level.  When electrons go the other way (down in energy level), light is emitted with that energy (i.e. the difference in the energy levels = the energy of the photon).

Now another fact of quantum physics is that the wavelength of a photon (a wave packet of light) is inversely proportional to it's energy.

Put these 2 facts together and you get the result you're asking about.

There is an amazing book that manages to explain Quantum Electrodynamics in relatively simple terms but without resorting to incorrect physics: "QED: The Strange Theory of Light and Matter", by Richard P. Feynman (a Nobel-prize winning physicist and a great teacher).  Check it out.

Answer 2

Electrons occupy a set of stable energy levels, or orbitals, and can transition between these states by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties.

plus to add on the answer, wave lengths are stopped or let through depending on the wavelength , this is the scenario with allot of materials such as long photon wavelengths trapped inside a green house . and short ones bouncing off.  

Answer 3

I believe it is because they the atoms are vibrating with a particular energy which when corresponds to the energy of the light that is incident on them - the atoms absorb the light.

  As for emitting light, it might be the be the same explanation the other way around! As in atoms are vibration at a certain frequency and hence emit light of a certain wavelength!

Answer 4

Simple Answer - There is a difference between the energy of their ground electronic state, where no electrons have been excited by an incoming photon, and their excited electronic state, where at least one electron has been moved to a higher orbital than it normally occupies.  The difference between these 2 states determines how much energy they absorb when they get excited and emit when they relax (go back to their ground state).  Energy of light absorbed or emitted =Planck's constant*frequency of light absorbed or emitted.  Frequency of light = speed/wavelength.

More complicated answer - Really requires a graduate level course in chemical physics.  Suffice it to say that the geometry of the orbitals plays a role and quantum mechanics gets in there a lot, too.  On the bright side, you get to learn what makes things the color they are.